An Introduction to Synthetic Gem Materials

Robert Weldon

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A synthetic gem material is one that is made in a laboratory, but which shares virtually all chemical, optical, and physical characteristics of its natural mineral counterpart, though in some cases, namely synthetic turquoise and synthetic opal, additional compounds can be present.

Synthetic gem crystals have been manufactured since the late s, and their production is often marked by a need for them in industrial applications outside of the jewelry industry. The first success was in producing synthetic ruby of faceting quality. Synthetic crystals are used in communications and laser technology, microelectronics, and abrasives. Because synthetics for jewelry applications can be “made to order” [i.e. consistent color and crystal shape] given the right ingredients, time, and the facilities to grow them, they are likely to be much less rare than natural gems of equal size, clarity, and saturation of color. Because of this, and because it is possible to confuse them with gems that are naturally occurring, there are strict guidelines regarding how they are marketed and sold.
 
In the United States, the Federal Trade Commission requires that any gem material produced in a laboratory be described in a way that leaves no doubt that it was not produced naturally. It is considered to be a deceptive practice if a synthetic gem material’s origin is not clearly disclosed throughout the distribution channel at the time of sale, from the manufacturer to the consumer. There are also a number of industry organizations such as the American Gem Trade Association (AGTA), the International Colored Gemstone Association (ICA), and the World Jewellery Confederation (CIBJO) that have formulated specific guidelines for their members regarding the disclosure of synthetic gems at the time of sale. In the last decade fewer new kinds of man-made gem materials have been marketed. This suggests that the repertoire of synthetic gem materials is close to reaching its limit in terms of the creation of new materials, but it is not limited in production which is still very significant. During the last century, researchers have developed a number of different ways to create these synthetic gem materials in the laboratory. Most of these methods fall into two major categories – melt or solution.

In melt processes, the chemical composition of melt is the same as the composition of the resulting crystal. In solution processes, the solution or melt has a different chemical composition than that of the resulting crystal. Constituents are dissolved in the solution or melt at high temperature, and the crystal forms initially on a seed crystal as the melt temperature is lowered. Some of the main synthetic processes include:

Flame Fusion or Verneuil process (melt process)

The first commercially successful synthetic gems were created by the flame fusion process. This process involves dropping powdered chemicals through a high-temperature flame, where it melts and falls onto a rotating pedestal to produce a synthetic crystal. Today it remains the least expensive and most common way to make gems such as synthetic corundum and spinel.

Crystal Pulling or Czochralski process (melt process)

Pulling emerged in the early s. In this process, nutrients are melted in a crucible and the synthetic crystal grows from a seed that is dipped into the melt, and then slowly pulled away from the melt as it grows. Gems synthesized by pulling include synthetic alexandrite, chrysoberyl, corundum, and garnet.

Flux growth (solution process)

Today some synthetic gems, such as emerald, ruby, sapphire, alexandrite, and spinel can be created through a flux-growth process. Flux is a solid material that, when melted, dissolves other materials in the same way that water dissolves sugar. As the dissolved chemical solution gradually cools, synthetic crystals form.
 
Growing a synthetic gem by the flux method requires patience and significant investment. Crystal growth can take up to a year, and the equipment is very expensive. But the results, especially when it comes to emerald, are well worth the time and effort.

Hydrothermal growth (solution process)

Like the flux process, the hydrothermal growth process is slow and expensive. But it’s the only method for successfully growing synthetic quartz. This process requires heat and pressure and imitates the conditions deep in the earth that result in the formation of natural gems. Nutrients are dissolved in a water solution, and then synthetic crystals form as the solution cools.
 
While the following list encompasses the commonly seen synthetics, over the years there have also been experimental synthetic gems. These include malachite, color change synthetic spinel and others. But because nature produces these products more readily, they are not often seen today. Some of the synthetic gems that are more frequently encountered include:

Synthetic diamond

These diamonds, grown in a laboratory, share most of the characteristics of their natural counterparts: they are essentially carbon.

Chemical vapor deposition (CVD) – diamond growth in a vacuum chamber due to a chemical reaction which releases carbon atoms that precipitate on diamond seed plates.
High pressure, high temperature (HPHT) – diamond growth from a melt flux which dissolves carbon at higher temperatures, and the diamonds form on seed crystals in a lower temperature portion of the growth chamber.

Synthetic corundum (widely available)

Synthetic corundum, which includes ruby and sapphire, can be made by the greatest number of processes. Because of this, synthetic corundum is available at many price levels, from very affordable to very expensive.
Ruby – in the late s, ruby became the first gem to be created in a laboratory by Auguste Verneuil. In , he announced the development of his flame-fusion process for synthesizing this beautiful gem. 
  Sapphire – some of the earliest examples of synthetic sapphires are in original pieces of art nouveau and art deco jewelry. Many synthetic sapphires are still made by flame fusion, but flux-grown sapphires have been available since the s. Flux-grown, pulled and hydrothermal synthetic sapphires can be very convincing substitutes for the natural gem. Color change synthetic sapphire, made to imitate alexandrite, has been popular since the early s. Induced inclusions caused star effects in some synthetic ruby and sapphire.

Synthetic emerald (widely available) and other beryls (rare)

Synthetic beryl is available in many colors including yellow, red, blue (aquamarine) and green (emerald). In the late s and s, Russia became a significant producer of these synthetic gems and is still a major supplier of hydrothermally grown gemstones such as synthetic beryl and synthetic corundum, along with others like synthetic diamond and synthetic alexandrite. Emerald – in the late s, scientists finally synthesized a commercially profitable flux-grown version of this desirable deep green gem. Hydrothermal synthetic emerald for jewelry came on the scene in .

Synthetic quartzes (widely available)

Gem-quality quartz, such as citrine, rose quartz, smoky quartz, and amethyst, is attractive. But natural gem-quality quartz is plentiful so scarcity is not the reason that researchers went to the trouble of developing a way to synthesize gem-quality quartz. The reason is that it plays a key role in technology. It can generate an electric current when it’s placed under pressure and can vibrate in precise response to alternating current. These virtues are put to practical use in watches, clocks, communications equipment, filters, and oscillators.

Amethyst: Lab-grown amethyst and other synthetic quartz varieties found their way into jewelry after being developed for industrial applications. The first hydrothermal quartz appeared in laboratories in the s. It wasn’t until World War II that synthetic quartz was widely available commercially.

Synthetic Spinel (widely available)

Early in the twentieth century, researchers trying to grow synthetic blue sapphire produced synthetic blue spinel by accident. Since then, synthetic spinel has been commonly used as a substitute for many natural gems. In the s, a new Russian-made, flux-grown synthetic spinel was introduced in a variety of colors including red, a color not widely available through the older flame-fusion process.

Synthetic Opal (occasionally seen)

In the s, the Gilson Company developed a three-step process to make convincing synthetic opal. First, microscopic spheres of silica are created through precipitation. Next, the spheres settle in acidic water for more than a year. Finally, a hydrostatic press consolidates the spheres without distorting the stacked arrangement that creates opal’s play-of-color.

Synthetic Alexandrite (rare)

Since there is not enough natural alexandrite to meet demand, various synthetics have appeared in the marketplace over the last few decades. Alexandrite has been synthesized by a number of different processes, including Czochralski, floating zone, and flux. Also, synthetic corundum with color-change is often used to imitate natural alexandrite. On rare occasions, synthetic color-change spinel may be encountered in the market.

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 -. 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 '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 . 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.

Synthetic Spinel

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 . 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.

Synthetic Quartz

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.

Synthetic Beryl

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 , but crystals weighing more than one carat could not be synthesized until . 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 .

Synthetic Emerald

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 , 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.

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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 , 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 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.

Diamond Imitations

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 . 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.

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