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New Gemstone Treatments - A Coming Crisis© Joel E. Arem PREFACE
Everything we “know” is nothing more than the most current theory that has yet to be disproved by new observations. A good theory or model will do two things: (a) it will immediately suggest the next experiments that need to be done to prove itself, and (b) it will make predictions of what the results of these experiments might be. Sometimes these predictions take a long time to be verified. It took more than 40 years for gravitational lensing, a phenomenon suggested in papers by Einstein and Zwicky in 1936 and 1937, to actually be observed (1979). But sometimes a theory accounts for all the available data, and is then used as a working model until a better one comes along. Events have occurred in the gemstone trade over the past few years that suggest the existence of new and undisclosed treatment processes. Treatments can be both a blessing and a curse. If properly disclosed, a gem treatment can win acceptance and provide the world with a much larger pool of useful cutting material than could otherwise exist. A good example is heating, ubiquitously applied to just about every gemstone that can withstand the process. Heat treatment is generally disclosed and is viewed as a positive application of useful technology, making gems cleaner, prettier and more saleable. But technology seems always to have a “dark” side. Proper use is good for everyone, but misuse can bring untold chicanery and misery. So gem treatment offers good news and bad news: the good news is that it may appropriately add considerable value to gem materials. The bad news is that it may inappropriately add considerable value to gem materials. The incentive is always great to cook a gemstone and NOT disclose the treatment, i.e., profit without accountability. This situation has prevailed for more than a century. Gemstone testing laboratories have, in past years, consistently lagged behind advances in treatment technology, because the return on funding for testing methods is never as great as the potential return to be gained by misrepresentation. This paradigm started to change only when the perfection of synthetic diamond and HPHT (high pressure/high temperature) treatment methods became widely known. After all, when diamond is involved, it (suddenly) becomes everyone’s problem! Laboratories scrambled to acquire instrumentation and testing methods to at least keep abreast of the treatment art. It has been an uneasy balance, with the stability of the entire jewelry industry at stake. Now we appear to be at a crossroads. Gemstones that were never previously suspect are appearing in the marketplace in huge quantities and are being sold as “natural and untreated.” Emanating from cutting centers such as Bangkok are stones that seem to be treated in ways never before observed. There may be emerging an entirely new generation of treatments, with 21st century technologies pitted against 20th century detection methods. Keeping up with this onslaught of new technology is a struggle, and frankly a battle the gemstone trade cannot afford to lose. The following discussion might be viewed as a report on one of the initial skirmishes in the coming war, with a few suggestions for a battle plan. A QUICK CRASH COURSE ON COLORThe universe is filled with radiation. Energy travels, in a vacuum, in the form of little particles called “photons.” But when traveling through anything else (gases, liquids, solids) energy seems to behave as ripples that we call waves. These waves look much like the ocean waves that strike a beach – there are high points called “crests” and low points called “troughs” that move to shore in an alternating sequence. The height of a wave (“amplitude”) indicates its strength. Compare the gentle waves you find on New Jersey beaches to the giants that wipe out surfers in Hawaii! The distance between successive crests is called the wavelength. You can easily imagine that if large waves hit a beach every 10 seconds, they will cause a lot more damage than if they strike the shore every minute or two. The energy carried by a wave depends on both its amplitude and frequency. If the waves hitting a beach have shorter wavelengths, it means that more waves will hit the beach per minute, carrying more energy. So short wavelengths mean waves with more energy; long wavelengths mean waves with less energy. The energy that fills the universe ranges from the unimaginably energetic short wavelength entities known as cosmic rays and gamma rays, to the long lazy waves that carry radio signals bouncing around our planet. The whole range of energies is called the “electromagnetic spectrum”. Our human eyes have evolved to “see” only a tiny portion of this spectrum, a range of wavelengths, more or less in the middle, that we call “visible light”. Each “color” that we can see is an even smaller piece of this tiny sequence of wavelengths. Red light has the longest waves we can see, and violet the shortest. The whole series is called the “visible spectrum” and is dramatically revealed when a rainbow forms, with colors appearing in the sequence red-orange-yellow-green-blue-violet. Wavelengths just beyond our ability to see at opposite ends of this spectrum are called infra-red (= heat energies) and ultra-violet (= sunburn energies).
But always remember that ‘colors’ are manifestations of specific levels of energy. Many objects in the universe also contain various types of energy, and these are capable of interacting with light waves in complex ways. For example, consider the atom. Most scientists accept the idea that atoms are the fundamental building blocks of all matter, and for the last century (more or less) the accepted “model” of the atom is visualized as a very small particle in the center called the nucleus, surrounded by a “cloud” of rapidly spinning negative charges called electrons. (We are all familiar with electrons. When they are moving in a wire they do a great job of heating up things they pass through, such as the filament in an electric light bulb, which then glows and gives off heat and light.) The atomic nucleus consists of neutral particles called neutrons, and positively charged particles called protons. In a “neutral” atom there are equal numbers of electrons and protons, so the positive and negative charges balance out. If an atom gains or loses one or more electrons, the result is an entity called an “ion”. If the atom loses electrons and is left with more positive charges in the nucleus than there are negative charges to balance them, it has a net positive charge and is called a cation (pronounced cat’-eye-on). The process of losing electrons is called “oxidation”. If an atom accepts electrons, it acquires a net negative charge (this process is called “reduction”), and the result is a negatively charged ion, called an anion (pronounced an’-eye-on). Ions are charged particles – atoms with either more or less electrons than there are protons in the nucleus. The exchange or sharing of electrons between atoms is called chemical bonding and is the force that makes atoms stick together. YES – this all has to do with color in gemstones – please bear
with me, and hang in! CRYSTALS AND CRYSTAL GROWTHCrystals are solids, which means that (a) they melt at a very specific and well-defined temperature, and (b) the atoms in them are arranged in very specific ways, with long-term “periodicity”. This means that the atoms in a solid are bonded together by electronic forces into specific, repetitive patterns. If you heat up a solid, the atoms start to vibrate. At a certain point the vibration is so energetic that the electronic bonds holding the atoms together in a fixed pattern actually break, and the atoms can move freely around each other, creating a liquid. The solid has melted. If you cool this liquid, the atoms start to move back together and, when the melt is sufficiently cooled, the bonding energy overcomes the heat-induced vibrations and the atoms arrange themselves into the same ordered (crystalline) structure as before. This so-called melting point is very specific and very reproducible, and is a basic property of any given solid material.
Glass is not a solid. This sounds very strange because if you hit glass with a rock it shatters, and it certainly seems solid enough. However, if you heat glass, it slowly gets hotter and hotter, turns red, and then flows. There is no clearly defined temperature at which the glass can be considered “melted”. Glass is amorphous – its atoms are not arranged in a periodic, extended geometric structure, but rather they are piled and jumbled together at random. There is no one specific temperature at which all the bonds between the atoms are broken, and certainly they are not broken all at the same time, which is why heated glass softens rather than melts. Glass is therefore called a “supercooled liquid” and is rigid, but not solid. Yes – terminology is very important, as we shall soon see. When a crystal grows, atoms that are moving around in a molten liquid or a hot solution stick together because of electronic bonding forces between them. If the temperature is too high, they un-stick almost immediately, and wander around again in search of other atoms to bond to. But as the temperature drops, the atoms do not move around as energetically. Eventually, atoms that stick together remain stuck together. These tiny clusters then start arranging themselves into a configuration that ‘minimizes the total energy of the system’, i.e., a crystalline structure that is characteristic of the atoms involved and the conditions of their surroundings. If you have ever played “Tetris”, you know that the object of the game is to twist and turn strings of blocks in order for them to lock into place without any spaces between them. This is exactly what happens in a growing crystal. The perfection of the final product depends on how quickly the crystal grows and many other factors. Think of the game of musical chairs. People wander in a circle around a string of open spaces and suddenly are asked to sit down when the music stops playing. Alas, in this game there is always one more person than there are chairs, and someone must be left standing. In crystals, falling temperature forces atoms to “sit” into positions determined by the growing structure (called a “lattice”). But what happens if an atom cannot find a “seat” in the lattice “when the music stops’”? In this case, it might wind up stuck between adjacent atoms, distorting the lattice. This kind of defect is called an “interstitial”. A hole in the structure where an atom should be is called a “vacancy”. These defects and stresses are important, as we shall see later.
BACK TO ELECTRONSThe atoms in a crystalline lattice form an array, like a playground “jungle gym,” a scaffolding with a specific shape and with specific distances between the junctions. There are many shapes of jungle gyms, and there are also many crystal geometries. The basic shapes of lattices are described in terms of crystal “systems”, and any given crystalline material is characterized by its lattice geometry and the specific atoms that form the periodic array. A mineral (or gemstone material) is defined, in fact, by its crystal geometry and constituent atoms. No two combinations are exactly alike.
What happens if a normally colorless crystal structure is invaded by an atom that doesn’t belong there? This is like a fat person playing musical chairs and trying to share a seat with another person when the music stops. It is a very uncomfortable situation. The presence of this foreign atom distorts the lattice and, therefore, the electronic field around its atoms. This distorted field no longer allows white light to pass through unchanged. Some wavelengths are absorbed, and the escaping light now has a visible color. This mechanism of lattice distortion creates what is termed a “color center”. Solids that are normally colored even when pure, such as malachite and azurite, are called idiochromatic (“self-colored”). In these materials the normal structure by itself creates the proper electronic environment for the absorption of specific light wavelengths. Materials that can become colored due to the presence of impurities are called allochromatic (“other-colored”). Allochromatic substances typically have color centers characterized by distorted electron fields that are created by the impurity atoms.
Many gem materials are colorless when pure (beryl, some tourmalines, corundum, etc.) and can acquire an enormous range of hues when impurity (foreign) atoms become trapped in their crystal structures. The actual color produced by an impurity is a function of BOTH the nature of the impurity AND the type of structure it invades. A perfect example is chromium, an element that can exist in crystals in several different oxidation states (i.e., different levels of positive charge). When chromium enters the structure of beryl, the result is an intense green hue (emerald); chromium in corundum turns it bright red (ruby). Sometimes two different kinds of impurities in a structure interact, via a mechanism called charge compensation. The new electron configuration thus produced may give rise to a new type of color center. A good example is the interaction of iron and titanium in corundum, giving us the lovely blue color that inspired the name sapphire. The same element in a structure can even exist in different oxidation states in different structural positions, and create several colors in the same material. Anything that distorts the crystal structure of a (normally) colorless material can potentially create a color center. Perhaps the best example is diamond. Clusters of nitrogen atoms will produce a yellow color in diamond. Boron impurities cause a blue color. But certain hues do not result from impurities. Rather, defects (misalignments) in the diamond crystal structure distort the electron fields and cause certain wavelengths to be absorbed, resulting in shades of red, pink and brown. Intense radiation can also be used to knock electrons out of their normal positions, and the resulting damage to the structure produces a color center. This is the process that gives us an endless supply of (treated) blue topaz. CREATING AND CHANGING COLORGemstone treatment is centuries old, and has long been an accepted aspect of the trade. Heating, in fact, is entirely responsible for the lovely blue color of zircon, and the intense saturated violet-blue that has made tanzanite so popular. But the “dark side” of technology is rapidly gaining ascendance. The production of chemically and physically altered gemstone materials is starting to reach epidemic proportions, and the health of the gemstone industry is now in jeopardy. The first step in dealing with this problem is to understand what is being done to alter gemstones, and the second is to discover ways to detect these treatments. Heating, as a process, is “low-tech” and relatively easy to understand. The same is true for gemstone “coatings” (thin layers deposited on gem surfaces), and even the physics of irradiation has been thoroughly investigated. But some new treatments that have arisen in the past few years are creating some concern, and perhaps even alarm. One of the most disturbing, and also one of the hardest to diagnose, involves the actual forced high-temperature penetration of chemical impurities directly into the crystalline structure of a gemstone. This process is generally labeled “diffusion”, but the terminology used to describe it is complex and has been hopelessly confused when verbalized outside of scientific circles. It is critical to get this vernacular on firm ground, or the gemstone trade will never be able to properly discuss it. This discussion that follows will allow us to examine and analyze massive problems that are now appearing in the gemstone field. DIFFUSION DEFINEDThe mechanism that has been suggested for the creation of both red “andesine” and the colors in treated topaz and some tourmalines and garnets is diffusion. The term “grain boundary diffusion” has been suggested as the correct descriptive terminology for what has been observed in some gem tourmalines, versus “bulk diffusion” to describe the process used for “andesine”. These terms can be confusing and are often incorrectly used. This can result in preoccupation with semantics rather than observations. In science, terminology IS important and it DOES matter what things are called. The simplest way to resolve this debate is to rely on internationally accepted definitions. A decade ago, the International Union of Pure and Applied Chemistry (IUPAC) organized a task force to create a lexicon of terms for solid state diffusion. The resulting recommendations were published in the journal Pure and Applied Chemistry in 1999. We may also rely on established reference works that provide widely accepted nomenclature standards. Here are the internationally accepted definitions for diffusion-related terms: Diffusion is defined (Glossary of Chemical Terms, Van Nostrand Reinhold, 1976) as: The mutual permeation of two or more substances due to the kinetic energy of their molecules, so that a uniform mixture or solution results. Lattice Diffusion (= bulk diffusion): A diffusion process which takes place through the bulk lattice of the crystal and excludes such mechanisms as short circuit diffusion along dislocations, grain boundary diffusion and surface diffusion. Short Circuit Diffusion (= grain boundary diffusion): Any diffusion process occurring via grain boundaries, surfaces or dislocations. Pipe Diffusion (= Short Circuit Diffusion): Diffusion along a dislocation. And finally, a grain boundary is defined (Glossary of Chemical Terms, Van Nostrand Reinhold, 1976) as: The surface separating two regions of a solid in which the crystal axes are differently oriented. The mathematical descriptions of diffusion contain several terms which are constants for specific materials, leaving the only significant variable as temperature. For all practical purposes, diffusion is a process that is dramatically enhanced as the temperature of the system rises. This, of course, makes perfect sense. Kinetic energy is the energy of motion, and in all substances (gases, liquids, solids) the movement of component atoms and molecules increases with rising temperature. Mixing of two substances is tremendously accelerated if the particles involved are buzzing around, rather than sluggishly crawling over and around each other. In the case of gases and liquids, molecular movement is already facilitated by their physical state, in which the molecules are free to move about. Solids are a different story. Misinformation often propagates because the visual images that have been created to describe certain phenomena are either wrong or oversimplified. I believe this has happened in the current dialogue. We are interested in diffusion in solids (which, by definition, are crystalline), and so we must have a proper visual concept of what a crystal looks like, on an atomic scale. It might be easier to do this if you can visualize the molecules (groups of atoms) in a liquid, such as water (a good visual reference might be a bucket of marbles). The molecules do attract each other because of electronic bonding forces, but kinetic energy allows them to slip and slide around each other. The molecules are still basically touching and packed together, without any open spaces. If a space did open up, the dynamic movement of the molecules would immediately close it. If the temperature of the system drops, the movement of the molecules slows down. Eventually (think "molasses") motion within the liquid is really sluggish, and the attraction of the molecules starts to overcome the kinetic energy that keeps them moving. At a very specific temperature (called the freezing point) the bonding forces overcome kinetic movement, and suddenly all the molecules instantly lock into a fixed pattern. This pattern extends identically through the entire mass of material, and the way the molecules arrange themselves depends on what kind of molecules they are, and certain rules having to do with pure geometry. We can describe these patterns in terms of boxes and lines, little diagrams that illustrate the shape of the pattern (like a single repeating element of a wallpaper design) and how the pattern is extended (with consecutive repeats, rotations, mirror images, etc.). But we cannot confuse the diagram with the reality; inside the crystal, there is no wallpaper – just lots of atoms, stuck tightly together in a fixed array, with certain symmetrical clusters that repeat themselves in all directions. This symmetry can be described mathematically. It turns out that geometric objects can only pack together in a limited number of arrangements, all of which were worked out centuries ago. Eventually, these arrangements were visually simplified into little diagrams, with boxes and lines representing the symmetry elements of the repeating patterns. Crystallography was born. The description of freezing obviously applies to a crystal that solidifies from a melted substance. Let’s go back to water. Water is molten ice. Ice is a crystalline solid and the geometry of its structure is wonderfully illustrated by the incredible hexagonal (6-sided) entities that we call snowflakes. If you could make an ice cube with total perfection, the pattern of water molecules in it would be unbroken in all directions, and the cube would display characteristic optical and physical properties, just like a crystal of beryl or spinel. If you poured maple syrup over this ice cube, the syrup would flow down the outside and not penetrate the ice at all, any more than it would penetrate a quartz crystal. But if you crush the ice in a blender, making thousands of tiny particles, each tiny bit would be randomly oriented – the long range crystalline structure of the original ice crystal would be destroyed. In this aggregate, the ice fragments would have crystallographic symmetry elements that are no longer perfectly aligned. Instead of a tightly packed periodic array, we now have a mass of tiny misaligned grains, with spaces between them. If you pour syrup over the crushed ice, the syrup can now penetrate along the grain boundaries and we have... a snow cone! Artificial diffusion-coloring has been observed in corundum and now, apparently, also in feldspars. How should gemologists classify the type of diffusion they are seeing? Some gemological journals have used the term “grain boundary diffusion”, but this term is only appropriate for describing color enhancement in certain specific gemstone materials. GRAIN BOUNDARY DIFFUSIONA well formed crystal is not made up of grains – it is a tightly packed array of atoms and molecules. Crystals seldom grow with complete perfection (outside of a laboratory) and most have internal irregularities. But these irregularities (dislocations, stacking faults, point defects, etc.) are not the boundaries between “chunks” of material that have somehow stacked up unevenly, like crates piled up in a warehouse. Rather, they are parts of the crystal where some atoms have slipped out of their normal positions, creating tiny irregularities but not huge open spaces. The long-range order is not affected, and adjacent parts of the crystal remain symmetrically aligned. Since, by definition, grains are “regions of a solid in which the crystal axes are differently oriented”, a single crystal is not made up of grains and is not filled with grain boundaries. The concept of “grain boundary diffusion” is not relevant in the case of most gemstone materials. Only a few gemstones grow from a melt. Most are created by the slow deposition of atoms and molecules as a hot, watery solution cools. But slow growth in a solution creates the same internal long-range perfection as slow cooling from a melt. The fact that gems are transparent indicates that their long-range internal crystalline structure has been maintained. If not, and the material was made up of small crystals jumbled together with NO long-range periodicity, light would not pass through unaffected, and the stone would be translucent or opaque (e.g., jade, chalcedony). In such cases there could well be grain boundaries, and the movement of fluids along them would be relatively easy (which is why jade and chalcedony can be dyed).
LATTICE, OR BULK DIFFUSION“Lattice diffusion” or “bulk diffusion” is the actual movement of atoms within a crystal lattice (i.e., a solid). But if crystals are made up of tightly packed atoms and molecules, how can anything move through them? The answer has two parts: temperature and defects. Remember the illustration of freezing water? Above the freezing point, the water molecules may be sluggish, but they do have relatively complete mobility. It’s like a packed crowd in Times Square on New Year’s Eve. You can jostle and squeeze through with a bit of effort, made easier if someone moves out of the way and creates a small open space that closes right behind you when you pass. Once the glittering New Year's Ball begins to drop, and people stop talking and moving around and stand still so they can watch it, getting through would be a much bigger chore. What if you could take a crystal and heat it up almost to the melting point (the same number as the freezing point, but coming up-temperature rather than down)? The atoms in the structure have not yet broken the electronic bonds that have locked them together into a repeating pattern. But they are still moving around much more energetically than when the crystal is cool. In our metaphor, the New Year's Ball has now reached bottom, the crowd has cheered, and the people in Times Square, though still standing in place, are getting ready to move. If you see an opening between two people, you can push your way through the crowd and keep moving through as long as there is a small space you can work your way into as you go. Crystals generally grow quickly enough that small irregularities do occur along the way. One of these is called a point defect – a place in the crystal structure where an atom is missing from where it should be (a vacancy), or merely a misalignment of adjacent atoms. At high temperature, but below the melting point, an atom can push its way into the lattice and into the defect; it can then continue to move through the structure by changing positions with atoms and finding additional point defects along the way. Point defects within a crystal also move around at high temperature, and are considered to be the primary mechanism of solid state diffusion. Since diffusion is almost completely dependent on temperature, you would have to get a crystal very, very hot into order to move atoms through any significant thickness of it. Ideally, maximum diffusion would take place just below the melting point, where the crystal structure remains intact but the atoms are moving very energetically and an impurity can work its way through. The problem is that only a handful of gem materials can melt when heated and then re-crystallize upon cooling (the best known being corundum and olivine) or withstand near-melting point temperatures without decomposing. For these materials, “lattice diffusion” is a correct term to describe the mechanism of coloration. But what about other materials, such as some tourmalines, that simply shatter or decompose when heated to a point that is still well below a temperature where diffusion to introduce a color is practical? PIPE DIFFUSION (ALONG A DISLOCATION)Crystals are seldom perfect. They contain a wide variety of defects that affect both their physical and optical properties. In addition to point defects, there are line defects where adjacent layers of atoms have shifted slightly "out of register." The misalignment can involve an entire plane of atoms as well. One type of plane separating misaligned units of structure is called a dislocation. What if an atom was trying to work its way into a crystal structure, and suddenly found a clear path for movement? Let’s go back to Times Square.
Imagine that there are several large tour groups visiting New York and are filling up most of the space at 42nd Street and Broadway. You are trying to move through the crowd but the tour groups are blocking your way. You notice that there is a lot of chatter and interaction between members within a group, but not so much along the boundary between the groups. It turns out to be not as difficult to move forward if you take advantage of the lower level of interaction along this boundary. If you looked down from a tall building you could probably make out the faint line separating the two groups, because the people would be interacting more frequently with others within their group than across the boundary. Now let’s imagine that the tour group leaders are all trying to do head counts. They ask all their tour members to line up in rows to make the count easier. In one group all the people turn facing uptown, and in the adjacent group all the people turn facing downtown. From your vantage point way up in the tall building, you would now easily see the line separating the groups, because at the boundary, even though the people were still packed together, they would have their backs to each other. In growing crystals there are often zones where adjacent layers of atoms suddenly switch orientation. These adjacent layers still fit nicely together, but the atoms may suddenly align themselves in a different direction. It is important to note that, in all such cases, the new alignment is symmetrically related to the old one, and as far as the crystal is concerned this is no big deal. It is as if a geometric wallpaper pattern had areas within it where piece of the pattern were turned upside down. The pieces of the overall pattern still fit nicely together because the patterns at the edges line up properly. But if you look at the wall from a distance you can see that a portion of the wallpaper ‘doesn’t look quite right’. It now consists of blocks that internally are pattern-perfect, but the way the blocks are stuck together they don’t continue the original pattern direction. A layer within a crystal where a shift in the symmetry of the atoms has changed the direction of growth is called a twin plane. Twinning is very common in minerals and is even visible macroscopically in materials such as ruby, sapphire, and feldspars. Twins are crystallographic regions within a crystal that are not aligned exactly the same way, but fit perfectly together because they are symmetrically related by the same geometric laws that define the crystal’s internal arrangement. From our point of view, a twin plane is just a zone within the material. From an atom’s point of view, it is a superhighway. In the Times Square analogy, even though the adjacent tour groups might still be packed tightly together, the line where people stood back-to-back would offer a trivially easy path for you to push through the crowd, with no significant interference. You could also move all the way through the crowd as far as the line separating the groups persisted. Twin planes provide an opportunity for impurity atoms to migrate through a crystal much more readily than via the slow, laborious process of playing “musical chairs" with lattice atoms, one at a time. Dislocations also provide a much easier path for diffusion, especially at high temperatures. IN SUMMARY:
The following discussion refers to feldspars, more specifically to brightly colored stones that started appearing in the marketplace only a few years ago. We will also look at possible new treatments of precious topaz and some tourmalines and garnets and propose several models for the origin of natural-colored labradorite (sunstone) that has been produced by Oregon for decades. TOO GOOD TO BE TRUEAbout eight years ago, I purchased a small lot of bright red gem rough, sold by dealers in Asia and represented to be “red andesine” from a "new and secret" locality, variously reported as "Congo" or "somewhere in China." This rough was somewhat expensive and was accompanied by a cover story that only about 20 kilograms of gem quality rough was available each year from this locality. Not long afterward, huge numbers of gemstones labeled “red andesine” appeared in the marketplace. The company primarily responsible for bringing this material to market also displayed large numbers of bright green stones, also labeled andesine. All the stones of each hue were almost exactly the same color, completely “eye-clean”, well cut and bright. Several associates and I were skeptical that such total uniformity could exist in this kind of gemstone, a plagioclase feldspar, considering the intense color zoning typical of its most comparable counterpart, Oregon sunstone. A close friend based in Asia then reported to me that someone from his company had visited the factory where these andesines were produced. I was told that this person had seen several enormous furnaces operating at very high temperature. He reportedly was told that the material being treated was cooked in three sequential stages, each lasting 30 days, to produce the desired result. No indication was given of the material being treated or the process used. I and several friends concluded that the “andesine” coming into the market in such uniform quality and prodigious quantities was most likely being treated in these furnaces, with a new and undisclosed process. Fast forward several years. The red andesine has now been sold in vast quantities. Suspicions have finally surfaced that the color of the material is not natural and that a treatment process has been employed without proper disclosure. A number of laboratories throughout the world have investigated, but their work has remained largely confined to scientific circles. Robert James, President of the International School of Gemology (ISG) reports publicly on the results of testing he performed and contracted for; these tests lead him to offer the opinion that the red “andesine” has been treated by “diffusion” of copper into plagioclase. Many people within the industry have expressed their suspicions about the authenticity of the “andesine” that started appearing in the marketplace more than five years ago. Significant research by trained scientists has now been done on this material. Shortly thereafter, James reported on his investigation of topaz and some tourmalines
and garnets, all emanating from Bangkok and showing possible signs of some kind
of new and strange treatment process. The process seemed to involve the addition
of color into the gems, pushed inward from the exterior of the (obviously not
eye-clean) stones along open tubes and zones of weakness. These results were presented
at a seminar in Tucson on February 6, 2009. A dialogue within the gemstone industry
regarding these reports has focused on the terminology used to describe
these observations, rather than on the possible reality and existence of the treatments.
This dialogue has confused the issue to the point where clarification is essential
if the problem of actually dealing with these treatments, rather than arguing
about how to describe them, is to be addressed. ANDESINEMany people within the industry have expressed their suspicions about the authenticity of the “andesine” that started appearing in the marketplace more than five years ago.. Significant research by trained scientists has now been done on this material. The observations all show the same features. The andesine seems to be more or less uniformly colored while retaining transparency, indicating some form of lattice penetration to produce genuine color centers (as in natural Oregon sunstone). However, coloration is more strongly visible along twin planes and within growth tubes, and even the transparent areas may have a splotchy appearance under magnification. Dr. John Emmett has commented that coloration by diffusion can be tremendously enhanced along crystal defects and twin planes, and we know that twinning is extremely common in plagioclase feldspars. Other researchers have observed whitish crusty material filling voids within some of the andesine rough, which has been interpreted as a flux designed to increase chemical mobility of a diffusion colorants. If all the available information is put together, we may surmise the following as the most likely scenario for most -- if not all -- “red andesine.” It is well known that significant quantities of transparent yellow labradorite occur in northern Mexico, and many tonnes have been extracted and sold. There is also new evidence of a mine in Mongolia that reportedly is capable of production on the order of 100 tonnes of pale yellow plagioclase. In both localities the coloring agent is iron and no copper is present in the native material. There have been reports that a treatment facility exists in which large quantities of this yellow feldspar are being heated to extremely high temperature (possibly over 1,200 degrees C.) for extended periods of time (reportedly 3 stages, each 30 days long), with copper (in some form) surrounding the rough being heated. At such high temperatures (near the melting point) it is likely that diffusion of copper into the plagioclase structure can occur. This diffusion would be greatly accelerated, with much greater depth of penetration, along any tubes and twin planes that might be present. Diffusion outward into the lattice along these defects would account for the irregularity and splotchy nature of coloration seen in many stones. The creation of true color centers (as in the case of Oregon sunstone) is indicated by uniform red (or green) color in large numbers of cut stones, some of them fairly sizeable. Reported zoning in a few samples, with green in the center and red outside, is the opposite of zoning observed in Oregon material. This reverse pattern is exactly what might be expected if copper was diffused into the material from the outside, versus what would occur during normal crystal growth in a magma, as I will propose later in this article. Dr. John Emmett, Dr. George Rossman (Caltech) and others have successfully diffused copper into plagioclase feldspar on an experimental basis, proving that the process is feasible. This in itself does not prove that the red and green andesine in the marketplace is treated, but supports the hypothesis that it could be. Naturally colored labradorite from Oregon has been known for decades. In all this time, and even still today, no locality has yet been identified that could produce rough that would yield the thousands of large, un-zoned, color-matched gemstones like those offered in the trade since about 2003. Oregon sunstone is often so intensely color zoned that it can be difficult to produce enough matching cut gems to make a necklace and bracelet, let alone 1,000 perfectly color-matched stones. The largest clean red Oregon sunstones appearing in the marketplace are typically on the order of 5-10 carats, and the number of these produced in a year of mining is limited. A tray of 50-100 clean, dark red gems, each over 10 carats, seems a bit suspicious. Research has conclusively demonstrated that the red and green andesine offered in the marketplace could have been produced by copper diffusion. In my view, the burden of proof has therefore shifted to the sellers, i.e., to prove conclusively that their stones are natural. This could easily be done by providing evidence of a deposit capable of yielding rough in sufficient quantity to justify the large volume of cut red and green gems being offered. Until this is done, the gemstone trade is justified in doubting the authenticity of this material. TOURMALINE, TOPAZ AND GARNETOn Feb. 6, 2009 at an open seminar in Tucson, Arizona, Robert James (ISG) offered photographic evidence of what he suspected and interpreted to be a new treatment process being employed to enhance the color of precious topaz, tsavorite garnet and some tourmalines. Observations on all three materials revealed a coating of what James calls “red crud” (thereby delightfully coining a new gemological term) on the outside of the stones (both rough and cut), from which bundles of tubes and needles filled with opaque colored material penetrate the gemstone. Some of the tubes are enveloped by a visible “halo” of color in the surrounding crystalline matrix. Observations at relatively low magnification (10-50X) do not resolve the nature of this halo. Robert James also observed markings on the exterior of a number of samples, which he interpreted as “burn marks” from strong radiation. James contracted with several independent laboratories to perform detailed spectroscopic analyses of the "crud," as well as chemical scans of the interiors of a number of cut gemstones received from dealers in Bangkok. The results indicated abnormally high levels of calcium, iron, and manganese. James also discovered that the crud is strongly magnetic. Tourmaline samples obtained directly from Nigerian and Mozambique sources did not show any unusual features. All the observed anomalies were restricted to gemstones obtained from Thailand. Similar patterns of treatment were observed on samples of tsavorite garnet and precious topaz. Diffusion is a function of temperature. Lattice diffusion may only be possible (within reasonable time spans) at extremely high temperatures. Diffusion along dislocations and twin planes is more likely to be a suitable mechanism in some practical applications. However, diffusion does not seem to be involved in the case of the stones that James believes are "treated" tourmaline. Impurity atoms in a crystal host may produce color centers, and distorted electron fields will absorb certain light wavelengths to yield visible colors. Atoms themselves have no “color”. If a gemstone displays a color that is confined to macroscopic defects, the mechanism involved may be something other than color centers. Although Robert James publicized his research by calling his observations proof of “diffusion”, the consistent presence of fissures and tubes (some quite large) in the stones he examined strongly suggests the introduction of an actual colorant, mixed with a chemical (flux) that would, when melted, allow it greater penetration into a crystalline host. Evidence of such chemicals was found as coatings on many of the tested stones. If a stone were irradiated prior to flux treatment, the consequent radiation damage might create a large network of micro-fissures and cracks that would be a conduit for a coloring agent far into the body of the stone. Heating would expand and open up pre-existing tubes and fissures, even further enhancing coloration. Some observations show a diffuse “cloud” of color inside a stone, emanating from a tube filled with red or yellow "crud". This suggests migration outward along microscopic cracks by material that had penetrated into the stone along the tube. In all these cases, the mechanism of coloration appears to be a simple variation of dyeing. Jadeite is routinely dyed to make it green; the dye is visible along grain boundaries and in cracks, especially ones extending inward from the surface. Agates (e.g. from Brazil) readily accept aniline dyes in all hues, to create dazzling bands of color. If enough fissures, tubes and cracks can be induced in a crystalline material such as topaz or a tourmaline, through radiation, heating, etc., a coloring agent could, with sufficient heating, and the addition of chemical fluxes to enhance penetration, be forced inward. It is critical to remember that, unlike olivine and sapphire, topaz and various tourmalines and garnets cannot be heated to a “melting point. These gems shatter and decompose when strongly heated – they do not melt. Lattice diffusion (as with andesine and labradorite) would not be possible, because the crystal structures would never relax sufficiently to allow significant atomic mobility. If this model is correct, the network of microscopic cracks accepting the coloring agent should be visible at high magnification. If high energy radiation is used to induce crystal damage, it is possible that some of the treated stones might show signs of residual radiation. Some gemologists have expressed skepticism about James' conclusions that the stones he is testing have even been treated at all. Additional work obviously needs to be done in many independent laboratories, ideally on some of the same gemstones analyzed by James, before these questions can be answered. OREGON SUNSTONEThe question of diffused red andesine raises another having to do with the origin of color in natural Oregon sunstone. All the zoned Oregon material so far recovered, from all known deposits, shows a similar pattern of color zoning: red core, green shell, colorless outermost layer. Some of the material has “Schiller”, a reflective phenomenon caused by myriad tiny particles of native copper within the host feldspar. To understand the cause of this color zoning we must visualize the actual geological formation of sunstones. Igneous rocks are created by the cooling of molten material called magma. Magma varies widely in composition, and so when it cools and solidifies, it can produce a huge variety of rock types. Certain melt compositions, rich in iron, aluminum, magnesium, chromium, and calcium, crystallize into dark colored rocks such as basalt, diabase, and gabbro. Other magmas might contain more silica, sodium, and other elements, resulting in granites. A rock is defined in terms of its overall chemistry, the minerals it contains, and the grain size of the crystals in it.
A dark magma might be extruded to the earth’s surface by a volcano and chill immediately into a very fine-grained rock (basalt) or even a glass (obsidian). The small grain size is a result of very rapid cooling. If the same magma were intruded into some rock layers deep underground, it might cool more slowly, resulting in a rock with the same composition and mineralogy as the basalt, but with larger crystals (due to slower cooling) and therefore given a different name: diabase. In a third example, consider a magma that remains underground for a long period of time and cools rather slowly. The first crystals to form in the molten material (with the right composition), the ones with the highest melting point, could be plagioclase feldspar. Given sufficient time, these crystals could grow to fairly large size (in the case of Mexican labradorite, as large as 4 inches across). If the magma found a crack leading to the surface, and was suddenly allowed to move upward and cool very quickly, the remaining melt would solidify into a typical basalt, a fine grained rock, but now containing scattered large feldspar crystals that formed during slow cooling, at depth. These crystals are called phenocrysts, and the rock itself is called a porphyry. The fine grained material around the phenocrysts is called a groundmass. All the Oregon labradorite deposits are porphyries. Each one potentially represents a different volcanic episode, and the entire area is part of an immense volcanic region called the Columbia Plateau. Creating the Plateau involved many periods of activity, and many individual lava flows. It might be that the flows producing sunstones all came from a single magma chamber with a unique composition. Or perhaps the copper might have been introduced in a later stage of activity. The answers are unknown, and this research has yet to be done. Copper is not a stranger to basaltic magma. There are basalts in Michigan that are characterized by large cracks and fissures entirely filled with metallic copper. These interlacing sheets of metal are sometimes broken free and make spectacular mineral specimens. If copper was present in the Oregon magmas, what would happen to it during the formation of a porphyry? It is probably safe to assume that if the copper was initially present in a magma chamber, it would be fairly uniformly distributed throughout the molten material. When the temperature dropped to a level where plagioclase feldspar crystals started to form, nucleation would be scattered randomly and each nucleating feldspar crystal would be surrounded by a “halo” of copper dissolved in the magma. The growing feldspar crystal would represent a better (i.e., lower energy) environment than the melt, and the copper present right around the tiny growing crystal would migrate to and into the growing crystal, depleting the concentration of copper in the immediate vicinity. Copper from further out would migrate towards the growing crystal but, by the time it reached it, the temperature would have dropped a bit. One would expect that the copper concentration in this region would be lower than at the center of the crystal, and it is possible that the copper incorporated at this stage might have a different oxidation state. A bit later the environment around the feldspar crystal would have been depleted of copper, and the next layer of growth would contain none. If the magma chamber cracked at this stage and the molten material shot towards the surface, we would find a basaltic groundmass containing phenocrysts of plagioclase feldspar, with high concentration of copper in the center and slightly lower concentrations moving outward, finally reaching a clear outer layer with no copper, or a negligible amount. Each phenocryst would have color zones representing the cooling history and the relative amount of copper in the original melt surrounding the crystal. In this model, irregular zones could indicate uneven amounts of copper in the original growth environment. An alternative hypothesis that could be proposed would suggest that the red and green colors are due to charge compensation between copper and a second impurity element that is present in the melt, where both impurities were incorporated simultaneously into a growing phenocryst. Depletion of one or the other would eliminate the charge transfer mechanism and change the color observed in the crystal. Research done so far does not seem to support this second theory. If the amount of copper in the melt was initially very large, and the magma was cooling rapidly, once the temperature dropped below a certain level there might be too much copper to be accommodated in the form of atoms trapped within the feldspar crystal structure. At this point the copper might exsolve as metallic grains, incorporated within the feldspar as it continued to grow, creating what we call “Schiller”. In this model Schiller would represent a growth episode characterized by high copper concentration and lower temperature. But in the final stage of crystallization we would still end up with feldspar phenocrysts in a basaltic groundmass. Extensive investigations of Oregon sunstone coloration were undertaken by Anne Hofmeister and George Rossman as early as 1985 but the results were published in the geological, rather than the gemological literature. Their conclusions were based on chemical and spectroscopic analysis, and pointed strongly to particle size as a major factor in the cause of observed coloration. Diffusion of copper within a phenocryst was also discussed, with the possibility that a temperature-dependant redistribution of the copper already trapped within a growing feldspar crystal could account for observed color zoning and appearance of Schiller. Many scientists have been working on feldspar coloration for years, but their conclusions are not widely known. Some have speculated that the copper in Oregon labradorite might have been introduced after the crystals formed, and created the observed coloration as a result of late-stage magmatic processes. The dramatic increase in popularity of Oregon sunstone has resulted in the extraction of large quantities of rough and the exploration of many new deposits. There is a huge mass of anecdotal information regarding the color distribution in the material. There is no shortage of speculation and theorization about the causes of the observed colors. The visual image of a mineral crystal forming in a magma, and becoming zoned as a result of chemical variation during crystal growth, is well established and readily verified by observation. I am simply presenting some ‘speculative working models’ for explaining color zoning in Oregon labradorite as starting hypotheses that can be tested with new observations, and modified or discarded completely with the production of new data or new availability of prior (perhaps unpublished) scientific work. I feel certain that, considering the growing interest in Oregon sunstone, any proposed model will be fully evaluated within the next few years. I am also presenting this discussion with hope that the methodology of such speculation is embraced by the gemological community as a way of providing a platform for debate, along the lines of normal scientific inquiry, as discussed in the Preface of this article. I hope that its publication results in discussion and even refutation, thereby bringing to light a wealth of new observations that would increase our knowledge database and assist in the creation of a new and improved working model. CONCLUSIONI have tried to present a theoretical basis for understanding the scientific issues underlying the observations of treatment processes that are plaguing the current gemstone industry. I hope that the industry will start to develop a more scientific approach in dealing with these problems. It is nearly impossible to evaluate data without constructing some kind of model or hypothesis to explain them. All of science is a history of proposing, testing and discarding theories, and progress is incremental. But the art of treating gems is a scientific process. It is only by adopting science as its ally can the gemstone industry maintain the ability to detect new processes, prevent misrepresentation, and thereby provide consumers with the confidence that will insure the long-term survival of the trade itself. Acknowledgements |
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