Understanding Gem Synthetics, Treatments, And Imitations, Part 4: Synthetic Gemstone Guide
Many varieties of gems besides diamond can be created in the laboratory. Our synthetic gemstone guide covers many examples and explains their manufacture.
18 Minute Read
Editor's Note: Many gemstones can be created in the laboratory and have a longer manufacturing history than diamond. This synthetic gemstone guide covers rubies, sapphires, emeralds, opals, and many more species and explains their fabrication processes. This five-part series of articles, "Understanding Gem Synthetics, Treatments, and Imitations," is a chapter from Dr. Joel Arem's forthcoming book, Gems and Jewelry, 3rd Edition. © Joel E. Arem 2011-2013. The International Gem Society (IGS) gratefully thanks Dr. Arem for his contributions to the field of gemology and for allowing us to reproduce this chapter.
Ruby and Sapphire
Ruby is aluminum oxide colored red by chromium. Synthetic ruby is often made by simply melting aluminum oxide that contains a trace of chromium. The resulting crystal has the same internal atomic structure as natural ruby as well as the same optical properties, hardness, and chemical composition. In fact, the only significant difference between this material and natural ruby is the place of origin, a laboratory, rather than deep within the earth.
Ruby and sapphire have long been considered two of the most desired and valuable gems. Natural material has never been available in sufficient quantity to meet world demand. It is therefore not surprising that their synthesis would be considered a worthy goal. The earliest experiments were those of Marc Gaudin in France in the mid-19th Century, although he never achieved the creation of gem quality corundum. In the mid 1880's, however, rubies appeared on the gem market that were initially thought to be natural, but which careful study showed to be manufactured products. Many of these rubies, known as "Geneva rubies," because it was thought that they were made near Geneva, Switzerland, were sold as natural. Just after the turn of the century another type of ruby appeared on the market. Termed "reconstructed ruby," this material was supposed to have been made by melting together bits of natural ruby. In recent years it has been demonstrated that such a process will not work, so these rubies must also have been synthesized from chemical raw materials.
A commercial process for manufacturing ruby was developed by Edmund Fremy of Paris. His rubies, however, all emerged in the form of thin plates. They could be manufactured cheaply in great quantity, and were sold widely for use in watch and instrument bearings. But they were too thin to provide large gems of fine color. In the last decade of the 19th Century, one of Fremy's assistants, August Verneuil, developed a new and different technique for synthesizing ruby. Fremy's method involved dissolving aluminum oxide in a molten salt and allowing ruby to crystallize from the melt by slow cooling. Verneuil's method has already been described.
Ruby can be made by adding a pinch of chromium to the aluminum oxide. Sapphire in various colors requires different combinations of metal oxides. It is interesting that the basic design of the Verneuil furnace hasn't changed much since the day it was first introduced in 1904. The furnaces can be automated so minimal staff can run many machines. Factories in Germany, France, and Switzerland may contain nearly 1,000 furnaces running at the same time, night and day. Massive production also exists in China, Thailand, and elsewhere. The output of such factories is measured in tons, rather than carats, and the cost of rough synthetic corundum can be as low as a few cents per carat. The crystals so produced, called boules, are cut in mass-production shops, sometimes by machine or by hand where labor is inexpensive.
There are other techniques for manufacturing corundum. Ruby for lasers can be grown by "pulling" crystals from a melt (Czochralski method), which can yield single transparent crystals inches across and several feet long. A more refined version of Fremy's method is also used to a limited extent. Today the method is called flux fusion, and the process yields ruby of fine color and clarity, although it is far more expensive than the Verneuil process. The flux process for ruby was perfected decades ago by Judith Osmer, but her trademarked "Ramaura" ruby is unfortunately no longer available in the marketplace.
Synthetic sapphire and ruby appear in a variety of commercial jewelry, such as class rings and birthstone jewelry. Usually a ring sold as "alexandrite" or "amethyst," where the label includes the quotes, is a synthetic stone. The so-called "alexandrite" sold to tourists throughout the world for a few dollars per stone, is actually synthetic corundum that has a color change reminiscent of true alexandrite. Colorless corundum, or "white sapphire," is manufactured in huge quantities for use as colorless gems and for bearings in electric meters, as well as for use in specialized electronic and military applications.
Synthetic Corundum Gemstones Grown By Flame Fusion Method
Star ruby and sapphire can be made by adding titanium oxide to the feed powder in a Verneuil furnace. As the corundum cools, the titanium oxide forms crystals of the mineral rutile within the host corundum. The rutile crystals are needle-like and orient themselves according to the symmetry of the corundum, which is hexagonal (six-sided), producing a six-rayed star when cabochon cut. The color range of synthetic star corundum is the same as that of the faceted gems. Synthetic corundum has distinguishing characteristics. The Verneuil process always produces curved growth lines, which are visible under magnification and with the correct illumination. No natural mineral ever displays such curved lines, called striae, and their presence is a guarantee of synthetic origin. Another characteristic of synthetics and glass is the presence of perfectly round bubbles, sometimes with a small tail, like a tadpole. Flux-grown rubies may show characteristic inclusions of the flux.
The first synthetic spinel was produced accidentally when some magnesium oxide was added to the feed powder in making synthetic Verneuil corundum. Spinel was not considered an especially valuable gem, however, so more than 20 years passed before synthetic spinel was used commercially in quantity. Natural spinels are not commonly encountered in the gem trade, but synthetic spinels are seen almost everywhere. These gems are widely used to imitate other gems that are considered more desirable, such as emerald, aquamarine, and tourmaline.
Synthetic spinel is normally made by the Verneuil process, and boules in a tremendous variety of rich colors can be grown. These colors are due to the addition of chemical impurities because pure spinel, as with pure sapphire, is colorless. In addition, spinel powder mixed with cobalt oxide and fused in an electric furnace produces a dense, deep-blue material that strongly resembles lapis lazuli. A spinel that resembles moonstone was introduced in 1957. Some spinel has also been made by flux fusion, and this material can be difficult to distinguish as synthetic.
Synthetic spinels may not show the curved growth lines seen in synthetic Verneuil corundum. But they can usually be identified as spinel (by refractive index), and the colors of the synthetic gems are usually sufficiently different from those of natural stones to warrant further testing.
Natural quartz is common and inexpensive. Yet synthetic quartz can be made in sufficient quantity and at low enough cost to make gem quartz manufacture worthwhile. Citrine, or yellow quartz, is colored by iron. Amethyst is made by adding specific impurities that produce a brownish color. A purple hue is created when this quartz is irradiated by a radioactive source. Colorless quartz is made in ton quantities for use in electronic applications but is seldom cut as a gem. Green quartz is also manufactured in limited quantity. Quartz is synthesized by the hydrothermal method. This is the way most natural mineral crystals form, in veins and cavities within the earth. While natural solutions are very dilute, and mineral crystals may take many years to form, in the laboratory the action is dramatically sped up.
Of the various beryl colors, by far the most valuable is the deep green of emerald. Experiments at emerald synthesis are known as early as 1848, but crystals weighing more than one carat could not be synthesized until 1912. Richard Nacken, who also developed the basic process for quartz synthesis, produced small emerald crystals using a hydrothermal process similar to that used for quartz. Later German experimenters succeeded in growing small emeralds of fine color, which were marketed as "Igmerald" by the I. G. Farbenindustrie conglomerate as early as 1934.
After World War II, Carroll Chatham of San Francisco introduced emeralds of large size and fine color. These were the result of research dating back to 1930, and apparently were made using a flux-growth technique. Synthetic emeralds have also been manufactured by the Linde Air Products Company, Pierre Gilson of Paris, Zerfass of Germany, and many others. The Linde emerald is grown hydrothermally using seed plates of colorless beryl. Gems are cut from the emerald that accumulates above or below the seed plate, so large thicknesses are required and are expensive to prepare. Large crystals of superb color are made by Gilson, and clusters of synthetic crystals are frequently offered for sale as jewelry items.
Synthetic emeralds can usually be distinguished from natural gems by the presence of characteristic inclusions. Natural emeralds have specific kinds of inclusions, which are often diagnostic of the country or mine of origin. Sometimes present are so-called "three-phase" inclusions consisting of a cavity filled with liquid, with a gas bubble and a crystal of sodium chloride or another salt inside. Synthetic emeralds do not generally display such inclusions, but may contain pieces of flux or other characteristic internal markings. Detection always requires the use of a microscope and, sometimes, additional gemological testing instruments.
Other Synthetic Gems
Pierre Gilson of Paris introduced three remarkable synthetic gems: opal, turquoise, and lapis lazuli. It is now known that the color flashes in precious opal are due to the regular accumulation of layers of minute spheres. Gilson duplicated this process in the laboratory, and his synthetic black and white opal is spectacular and natural looking. Careful tests may be required to distinguish it from natural opal.
Gilson turquoise resembles the finest Persian turquoise. It is extremely uniform in color and texture and available in cut stones or rough blocks. Under the microscope this turquoise consists of an aggregate of tiny spheres of uniform size, allowing it to be readily distinguished from natural turquoise.
Synthetic alexandrite is not corundum with an alexandrite-like color change, but rather a true synthetic chrysoberyl containing suitable impurities. The color change is green to red, resembling Russian alexandrite. Large cut gems are available, but the cost is high for a synthetic - in the range of synthetic emerald.
Synthetic rutile, chemically titanium oxide, appeared on the market in 1948, under various trade names. Natural rutile is nearly always opaque or a very dense, deep red color. Synthetic rutile can be made by the Verneuil process in a variety of colors, including brown, yellow, red, and blue. Completely colorless stones cannot, however, be produced and always have a tinge of yellow. The colored varieties were seldom seen in the gem trade. Rutile is too soft to be useful as a gemstone (hardness 6-6.5 on the Mohs scale). But its dispersion is about six times higher than that of diamond. Cut rutile therefore blazes with myriad colors. The color display is so dazzling and breathtaking that cut rutile loses credibility as the diamond it is supposed to imitate. There is simply too much fire to be "real." Cut rutile, sold as "Titania," is occasionally still available, but long ago lost its popularity to more suitable diamond imitations, especially cubic zirconia.
Some other synthetic materials that have natural analogs include: scheelite (calcium tungstate); apatite (calcium phosphate); wulfenite (lead molybdate); proustite (silver arsenic sulfide); gahnite (zinc aluminate, a variety of spinel); periclase (magnesium oxide); fluorite (calcium fluoride); zincite (zinc oxide); bromellite (beryllium oxide); feldspar (aluminum silicate); zircon (zirconium silicate); phenakite (beryllium silicate); and sphalerite (zinc sulfide). All of these have probably been cut as curiosities for gem collectors.
Characteristics of Synthetics
Each crystal-growing method is somewhat unique and uses different equipment, chemicals, containers, and so forth. Natural crystals also grow in a wide variety of physical and chemical environments. Every crystal-growth process leaves its mark on the growing crystal in the form of color zones, inclusions, surface shapes, and so forth. At any given moment during the growth of a crystal, the surface is characteristic of both the environmental conditions and the growth process. As material is added to this surface, the newly added layer becomes the new outermost layer. We can therefore say that crystal growth is characterized by a succession of surfaces, and a crystal's history is documented by the record of its surfaces in a way very analogous to tree rings. Moreover, crystal growth environments are seldom absolutely pure. Contaminants may enter the growing crystal and be trapped within it; these may be chemical impurities or sometimes crystals or bits of foreign substances. Even the kinds of surfaces bonding the crystal during growth are characteristic of the growth process. Many of these features are visible, with correct illumination, under a microscope. Microscopy is therefore unquestionably the most powerful working tool for the gemologist who wishes to distinguish between natural and synthetic materials. This is especially important because most homocreate* materials have properties almost identical to their natural counterparts or properties within the range observed for the natural substances. Easily measured properties, such as refractive index, specific gravity, emission spectrum, optic sign, even color, are not always definitive in identifying homocreates.
Also, the range of materials and growth methods used today is so vast that considerable experience is required to make positive identification. Crystal inclusions may be so small that magnifications up to 50x or more are required to see them properly; such inclusions may be the only proof of natural versus synthetic origin. Some gemstones, such as amethyst and citrine, are extremely difficult to distinguish, and in some cases identification is impossible. The value of a gemstone in the marketplace is largely a function of rarity, a feature not typical of synthetic stones. The marketplace has expressed great concern over the issue of non-detectable synthetics and their impact on gemstone prices. To be sure, a non-detectable homocreate would be a serious problem if no tests could be developed to recognize it. It must be realized that pecuniary interests drive all markets. In the past few years the emphasis has been heavily weighted toward making good homocreates since the monetary return for success is immense and far greater than the reward for developing new detection methods. In other words, you can make a lot more money fooling the marketplace with a newly created gemstone than by selling instruments to detect these gemstones. The gemological field has a lot of catching up to do.
Following is a brief summary of the characteristics typical of various homocreate and synthetic gems produced in laboratories. It must be remembered that overlap in features is common, and single characteristics, with a few notable exceptions, are seldom sufficient for positive identification. Vapor growth is not discussed in detail because this method is not of major importance for gemstones.
Melt Growth: Some techniques, such as Bridgman-Stockbarger, would leave virtually no identifying characteristics. Czochralski and Verneuil crystals, however, have such rapid growth rates that certain features become apparent. Melt growth is typified by rounded surfaces versus the plane surfaces found in natural crystals. These are observed as faint (sometimes distinct) lines visible with correct lighting. If you want to see what these so-called curved striae look like, take a telephone book, bend it slightly, and look at the side with a 2x magnifying lens. This image of a stack of gently curved parallel lines is very similar to the series of parallel bands (actually the series of former surfaces of the growing crystal) seen in most Verneuil crystals. Curved striae are instantaneous proof of synthetic origin. They are never found in natural crystals. Pulled crystals, however, normally do not display such features. Instead, we may find tiny metallic inclusions that separated from the container that was used to grow the material (for example, platinum) and occasional round bubbles. Round bubbles or tadpole-shaped bubbles with curved tails are also typical of melt-grown crystals and are positive identification features.
Solution Growth: This is a real gray area since natural crystals typically grow in hydrothermal solutions. The highest percentage of misidentified homocreates probably falls into this category. Experience, a good, high-powered microscope, and a suspicious nature are likely to be a gemologist's most useful tools. Multiphase inclusions (gas/liquid) are found in both natural and solution-grown crystals, although three-phase inclusions (solid/liquid/gas) have not yet been duplicated in the laboratory in sufficient numbers to create identification problems.
Flux Growth: The most commonly observed feature is flux particles trapped in the synthesized crystal; these may resemble breadcrumbs or comets, clouds of dust-like particles, twisted veils, and so forth. No single feature may prove diagnostic in some cases. Rather, the gemologist must rely on experience and a broad pattern of features for identification. Even so, it is common for some stones to defy analysis. The best rule of thumb is when in doubt, don't buy. If you pay the price for a fine quality natural stone, be sure it can be proven so.
The appearance of rutile on the market started a hunt for crystals that, when cut, would resemble diamonds. A problem existed with rutile because of its unavoidable yellowish color. This problem was solved with the introduction of strontium titanate in 1955. Closely related to rutile, strontium titanate's advantage was its pure white color, with no yellowish tinge. Its hardness, however, 6 on the Mohs scale, is still too soft to be very useful in rings. Another advantage of strontium titanate is its dispersion, which, though very high (four times higher than diamond), is lower than that of rutile and thus more realistic. Cut gems do resemble diamonds very strongly, especially when they acquire a slight oily film, which further cuts down the dispersion. Strontium titanate does not exist as a natural mineral. Its softness left an opportunity for a still better diamond imitation material.
This marketing gap was filled by a material called YAG, an acronym for Yttrium Aluminum Garnet. YAG is one of a family of so-called "garnets," named because their internal atomic structure is like that of the natural garnets. But here the similarity ends, because YAG and its brothers with similar rare-earth chemistries, such as GGG (Gadolinium Gallium Garnet), do not occur in nature.
YAG was originally grown for use in lasers, which is still its major application. It was accidentally discovered that, when properly cut, YAG strongly resembles cut diamond, even though its dispersion is relatively low. In addition, the hardness of YAG is about 8 on the Mohs scale, so cut gems are durable and do not scratch easily. YAG can be colored richly by impurities, and cut stones may resemble emerald, kunzite, sapphire, and other gems, although YAGs are too brilliant and hard to be convincing substitutes for most gems. The newest and most important imitation diamond material is cubic zirconium oxide, or "zirconia." This material is as hard as YAG (8.5), but has a much higher dispersion. In fact, the dispersion of zirconia is slightly higher than that of diamond, giving extremely realistic "fire" to cut gems. Such stones are lively, hard, and durable, and virtually indistinguishable from diamond to the untrained eye. Small zirconia gems in jewelry settings sometimes pose severe detection problems for the jewelry trade. Zirconia sells for several tens of dollars per carat or less, offering the consumer a stone with much of the beauty of diamond at a fraction of the diamond price. Other gem materials created solely in the laboratory include lithium niobate, sometimes sold as "Linobate," with a Mohs hardness of 6, yttrium aluminate, and potassium tantalate-niobate, whose chemical acronym is KTN. Few cut gems of these materials have appeared on the market but if encountered they could pose a real detection problem for the average jeweler.
Doublets and Triplets
Doublets and triplets are composite or assembled stones, with either two or three layers. The possible combinations of materials used in making such gems are many, and you'll find a wide variety of composites in the gem trade. Typically, composite stones are created to display good color or create a hard outer surface. Genuine stones are rarely used for the bottom portion, although doublets of diamond on sapphire or spinel are known.
You'll commonly see doublets with garnet tops and glass pavilions. The garnet portion is so thin that the stone's color is dominated by the color of the glass, which may be blue, green, pink, red, or blue-green. Colorless doublets are also made, as well as doublets with hollowed-out, liquid-filled crowns cemented to colorless bases.
In past years, factories created doublets with colorless synthetic sapphire or spinel crowns and strontium titanate bases. These worked as effective diamond imitations, in which the softer titanite base provided dispersion color and the harder top provided protection from wear.
Soude emeralds are made by cementing together components of colorless quartz or synthetic spinel, using a green cement to give color to the gem. Such stones are easily detected if unset and viewed from the side.
Other kinds of doublets include those with quartz top and glass base, or with quartz top and colored-glass base.
Opal doublets consist of slices of opal mounted on a backing of onyx, ceramic, or opal. An opal triplet has an added quartz top. Ingenious jadeite triplets have been made consisting of translucent jadeite top and bottom, but with the upper portion hollowed out and a mass of the same material carefully fitted and glued in with a green-dyed cement. The resulting stone sometimes resembles the finest "Imperial" jade.
* A Note From Donald Clark: Dr. Arem's article, "Understanding Gem Synthetics, Treatments, and Imitations," is a wonderful piece. I have a great deal of respect for Dr. Arem. His Color Encyclopedia of Gemstones is the best reference of its type. He once helped me with a difficult identification. I didn't expect a personal letter from him and was pleased that he would go out of his way to help me. This was before the existence of the IGS. However, you should be aware that he defines the words "synthetic" and "homocreate" in a manner inconsistent with our industry standards. In my article, "How Gems Are Classified," I define "synthetic" as "materials that duplicate their natural counterparts" and "homocreate" as materials that have "no counterpart in nature," in accordance with the Gemological Institute of America (GIA).
Joel E. Arem, Ph.D., FGA
Dr. Joel E. Arem has more than 60 years of experience in the world of gems and minerals. After obtaining his Ph.D. in Mineralogy from Harvard University, he has published numerous books that are still among the most widely used references and guidebooks on crystals, gems and minerals in the world.
Co-founder and President of numerous organizations, Dr. Arem has enjoyed a lifelong career in mineralogy and gemology. He has been a Smithsonian scientist and Curator, a consultant to many well-known companies and institutions, and a prolific author and speaker. Although his main activities have been as a gem cutter and dealer, his focus has always been education. joelarem.com
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