A "simulant" gemstone is a synthetic material that is different in chemistry and properties than the original but looks similar to the original. One of the most common simulants is glass, rhinestones are usually glass with colored foil backing to give them color, and are used in clothing and jewelry to look like semi-precious and precious stones. Glass is a generic simulant for may real stones.

The list of stones and examples shown at the bottom run a wide range of both synthetic and simulant stones. The synthetic stones are often better than the real ones in terms of purity and color, and can be difficult to tell apart without the aid of sophisticated instrumentation. Most of the simulant stones are poor substitutes and easy to differentiate even by eye.
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Synthetic corundum: there are three processes now for creating synthetic corundum, the Verneuil process, the Czochralski process, and the 'recrystallization process". The Verneuil process can be seen in the following [animation]and is described below in more detail. It is basically the formation of corundum in a flame. It is the oldest commercial process of the three. The widest variety of colors is creating using this process. There is substantial internal stress in the boule that is create and they must be split to reduce the stress, hence they cannot produce long, stress free material this way.

The second process, the Czochralski process, is the formation of the corundum from a melt. It is similar to the flame process in that it also is a "drawing process" involving growth on a seed Crystal that is slowly moved over time. The real advantage is that the stress is released during the forming process by rotating the growing boule. There is an [animation] of it also. Although many colors can be produced in this process the most common ones are pink and red as it used to produce ruby-rods for lasers.

The final process (not animated) is a recrytallization process, where the corundum is slowly grown in a mixed oxide melt solution. In some cases the starting components are actually low quality (impure) rubies and they are first dissolved in the melt, then recrystallized. The process of slow and controlled crystallization is in itself a purification process. Unlike the above processes where the synthetic has a unique rough shape, the recrystallization processes produce crystals and are sometimes used to sell as real stones at some mining locations in Asia. A fraudulent practice.

Another synthetic sapphire is Linde Star sapphire which was created with excess rutile in the mix and produced nearly perfect bright stars. It has been produced in a number of colors, shades and sizes.
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Synthetic quartz of very high purity has been created by the recrystallization process and is now available in all of its natural colors as well as several "unnatural" colors. Quartz has been used extensively in the electronics industry for oscillators and timing devises. Hence a readily available supply was needed and a method created to fabricate it. It is also used for high-temperature windows and containers and in optic systems.

The process is illustrated in the [ animation ] and can be used to create rock crystal (clear), amethyst (various controlled shades), citrine (various shades), ametrine has also been made in the laboratory, blue quartz (multiple shades - not natural), and some green quartz (again not natural colors).

The quartz is chemically and structurally identical to the real thing, and is very difficult to differentiate. Manufacturers can produce quartz that is virtually indistinguishable form the real thing. Twining was once thought to be a way to tell it apart, but twined seed crystals can be used in its production and this method is less viable than before.
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Cubic Zirconia: this is a simulant and not a gem synthetic as there is no equivalent natural product. Cubic zirconia (CZ for short) was created in a search for laser material and high temperature ceramics, but it was found to have excellent optical qualities that more closely mimicked those of diamonds.

It quickly replaced other diamond substitutes that were on the market. As the method of production improved ( the skull melting process), a wide variety of colors became available and the quantity sky-rocketed. An [ animation ] can be viewed showing the process. The unique thing about CZ is that it has such a high melting temperature that there is nothing else than hold it, except itself. Hence the process involves the "in-situ" creation of the vessel that holds the melt.

It has an excellent hardness at about 8.5 and good optical dispersion. It is relatively inexpensive to create and can be produced in reasonable sizes. It is the only diamond substitute that is inexpensive and looks remarkably good to the untrained eye.

Although not shown in this web site, there is another excellent diamond substitute available, but it is too costly for most use. Synthetic silicon carbide has been produced for may years a an abrasive material. It has a high hardness and is used for nearly all cutting when diamond itself is considered too expensive.

For many years the material produced was black and opaque. Eventually someone found a way to make the SiC in a pure form. It is now called Moisonite in the gem trade. Moisonite is chemically pure silicon carbide with a hardness of 9.25 (second only to diamond) and can now be created transparent and colorless. The problem is the cost, a one carat stone can cost about $400. A one carat CZ can be obtained for less than $5. ( http://www.moissanite-stardust.com/index.html )

Other higher priced simulants now include CZ that has been coated with diamond. The exterior has nearly the same hardness as diamond and the exterior coating produces more fire in the stone. They can fetch about $200 per carat. Again a nice material but to the untrained eye, they are not likely worth the extremely high cost. For further information see ... ( http://www.betterthandiamond.com )

True synthetic diamonds are also now reaching the market for cut stones. Originally General Electric produced thousands of carats of diamonds, but mainly for use in cutting tools and as abrasives It was not economical to produce stones for cutting purposes as the high purity was an issue. More recently there have been several announcements about the production of larger sized synthetic diamonds suitable for cutting. Two viable alternatives have surfaced, one from Russia and one from Florida (Gemesis). For further information see: ( http://www.gemesis.com/ ) The cost is about 15% below the equivalent natural diamond.
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Emerald - synthetic emerald has been produced in a couple of labs, the Gilson Emerald and the Chatham emerald are both made by recrystallization processes. Although the exact technique is still a trade secret, it is known that the process can take from 7 months to a year for the controlled growth, and that the emerald must be grown in platinum crucibles. ( http://www.chatham.com/ )

Red beryl has also been created using the same general process.
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Fiberlite: this is actually fused and colored fiber optic cable. So the question was, "What do yo do with old unused fiber optic cable?". and the answer appears to be color it then fuse it together and sell it a carvings or gemstones. I guess you could call it a simulant for any cateseye gemstone, but the colors tend to be pretty much fake and not like anything natural.

Very inexpensive and some "rough" does make its way to market, but most of he material is found in medium size sculptures or spheres.
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Goldstone: kind of simulant for aventurine or maybe sunstone. The material is actually a glass with precipitated or dispersed copper particles throughout the material. Originally the method of making it was attributed to an Italian Monk. It has more recently been licensed to a number of providers and much of it seems to come from China now.

It gets its name from the gold-brown appearance of the original material. It has now been made in blue, violet, and green varieties too.
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Spinel has also been produced in the Verneuil process, it is simple mixture of aluminum oxide with magnesium oxide. Unlike the flame produced corundum the spinel has far less interior strain built up during the creation cycle. Thus full boules of spinel can be recovered and do not require splitting to reduce strain. It too has been produced in a wide variety of colors.
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"crab fire agate" - I saw this material listed on e-bay and there as been some controversy about the use of the name "fire agate" as it is obviously not related to true fire agate. I was intrigued and so purchased 3 "bead/cabochons" and the first thing I did was to cut one in half and grind one site flat to look at the homogeneity of the stone. Well, I listed it under "synthetics" - that should be a hint!