I have been recently asked if I can again list what Diamond Simulants are on the market today and how do you tell if you have a simulant or real diamond. The problem is, new ones are appearing at regular intervals and it is very hard to stay current. Well the oldest test, but not totally conclusive is to run the diamond (or rock) across a piece of glass and if it scratches it, it is probably a diamond. As seen on TV you could hit it with a steel hammer ,and if it isn't ruined, it may be a diamond. Yea, but what jeweler or vendor in his or her right mind is going to let you do either of those two tests. So your best bet is to completely trust your gemologist or jeweler that what you are being sold is genuine. And if you bye a diamond from someone you don't know, you probably better make sure you can return it, because if you have it tested and find it is not a diamond, you want some recourse on what you spent. And of course the best saying is, "If it is too cheap for what it is suppose to be, it probably isn't what it is advertised to be!" The following is a chart of some simulants if you want to test for yourself..
Common Diamond simulants and their gemological properties
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Material. Formula..Refractive Index.Dispersion.Moh's Hardness.Density.Thermal.Cond
Diamond......C..................2.417..............0.044...............10................3.52....Excellent
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Artificial Simulants:
Glass.......Silica...............~ 1.6................> 0.020.............6.............2.4-4.2........Poor
Artificial Simulants:
Glass.......Silica...............~ 1.6................> 0.020.............6.............2.4-4.2........Poor
White Sapphire .Al2O3....1.762 ................0.018.............9.................3.97.........Poor
Spinel...MgO·Al2O3.........1.727.................0.020............8................~ 3.6..........Poor
Rutile......TiO2.................2.62 ..................0.33.............~ 6................4.25.........Poor
Strontium Titanate.SrTiO3.2.41................0.19.............5.5...............5.13.........Poor
Spinel...MgO·Al2O3.........1.727.................0.020............8................~ 3.6..........Poor
Rutile......TiO2.................2.62 ..................0.33.............~ 6................4.25.........Poor
Strontium Titanate.SrTiO3.2.41................0.19.............5.5...............5.13.........Poor
YAG.......Y3Al5O12.......1.83....................0.028.............8.25...........4.60..........Poor
GGG.... .Gd3Ga5O12..... ..1.97.................0.045.............7................7.02...... ..Poor
CubicZirconia.ZrO2.......~2.2....................~0.06..........~8.3..............~5.7..........Poor Moissanite....SiC..............2.648................0.104...........8.5...............3.2..........High
CubicZirconia.ZrO2.......~2.2....................~0.06..........~8.3..............~5.7..........Poor Moissanite....SiC..............2.648................0.104...........8.5...............3.2..........High
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Natural minerals that (when cut) optically resemble white diamonds are rare, because the trace impurities usually present
in natural minerals tend to impart color. The earliest simulants of diamond were colorless quartz, topaz and beryl (goshenite); they are all common minerals with above-average hardness (7–8), but all have low RIs and correspondingly low dispersions. Well-formed quartz crystals are sometimes offered as "diamonds," a popular example being the so-called "Herkimer diamond". Topaz's SG (3.50–3.57) also falls within the range of diamond.
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From a historical perspective, the most notable natural simulant of diamond is Zircon.
It is also fairly hard (7.5), but more importantly shows perceptible fire when cut, due to its high dispersion of 0.039. Colorless zircon has been mined in Sri Lanke for over 2,000 years; prior to the advent of modern mineralogy, colorless zircon was thought to be an inferior form of diamond. It was called "Matara diamond" after its source location. It is still encountered as a diamond simulant, but differentiation is easy due to zircon's anisotropy and strong birefringence (0.059). It is also notoriously brittle and often shows wear on the girdle and facet edges. Much less common than colorless zircon is colorless scheelite. Its dispersion (0.026) is also high enough to mimic diamond, but although it is highly lustrous its hardness is much too low (4.5–5.5) to maintain a good polish. It is also anisotropic and fairly dense (SG 5.9–6.1). Synthetic scheelite produced via the Czochralski process is available, but it has never been widely used as a diamond simulant. Due to the scarcity of natural gem-quality scheelite, synthetic scheelite is much more likely to simulate it than diamond. A similar case is the orthorhombic carbonate cerussite, which is so fragile (very brittle with four directions of good cleavage) and soft (hardness 3.5) that it is never seen set in jewelry, and only occasionally seen in gem collections because it is so difficult to cut. Cerussite gems have an adamantine luster, high RI (1.804–2.078), and high dispersion (0.051), making them attractive and valued collector's pieces. Aside from softness, they are easily distinguished by cerussite's high density (SG 6.51) and anisotropy with extreme birefringence (0.271).
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Desert Diamonds are also known as Saudi Diamonds, or sometimes Qaysumah Diamonds.
They are natural, semi-precious stones from the same micro-crystalline family as Amethyst, smoky Topaz and Citrine. These unique, high-grade stones have physical properties that produce the appearance of the expensive carbon-based diamonds! DESERT DIAMONDS are FOREVER, just like their carbon diamonds cousin. They are guaranteed not to discolor or break with age. This is the huge advantage of owning Dessert Diamonds vs. cubic zirconia. Compared with the carbon-based stones, which have a hardness factor of 10 on the Mohs scale, the Desert Diamond are only rated at a mere 7.0 on the Moh's Scale!
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Due to their rarity fancy-colored diamonds are also imitated, and zircon can serve this purpose too. Applying heat treatment to brown zircon can create several bright colors: these are most commonly sky-blue, golden yellow, and red.
Blue zircon is very popular, but it is not necessarily color stable; prolonged exposure to ultraviolet light (including the UV component in sunlight) tends to bleach the stone. Heat treatment also imparts greater brittleness to zircon and characteristic inclusions. Another fragile candidate mineral is Sphaerite (zinc blende). Gem-quality material is usually a strong yellow to honey brown, orange, red, or green; its very high RI (2.37) and dispersion (0.156) make for an extremely lustrous and fiery gem, and it is also isotropic. But here again, its low hardness (2.5–4) and perfect dodecahedral cleavage preclude sphalerite's wide use in jewelry. 
Two calcium-rich members of the Garnet group fare much better: these are grossularite (usually brownish orange, rarely colorless, yellow, green, or pink) and andradite. The latter is the rarest and most costly of the Garnets, with three of its varieties—topazolite (yellow), melanite (black), and demantoid (green)—sometimes seen in jewelry. 
Demantoid (literally "diamond-like") especially has been prized as a gemstone since its discovery in the Ural Mountains in 1868; it is a noted feature of antique Russian and Art Nouveau jewelry. Titanite or Sphene is also seen in antique jewelry; it is typically some shade of chartreuse and has a luster, RI (1.885–2.050), and dispersion (0.051) high enough to be mistaken for diamond, yet it is anisotropic (a high birefringence of 0.105–0.135) and soft (hardness 5.5). Discovered in the 1960s, the rich green tsavorite variety of grossular is also very popular. Both grossular and andradite are isotropic and have relatively high RIs (ca. 1.74 and 1.89, respectively) and high dispersions (0.027 and 0.057), with demantoid's exceeding diamond. However, both have a low hardness (6.5–7.5) and invariably possess inclusions atypical of diamond—the byssolite "horsetails" seen in demantoid are one striking example. Furthermore, most are very small, typically under 0.5 carats (100 mg) in weight. Their lusters range from vitreous to subadamantine, to almost metallic in the usually opaque melanite, which has been used to simulate black diamond. Some natural spinel is also a deep black and could serve this same purpose.
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Because Strontium Titanate and glass are too soft to survive use as a ring stone,
they have been used in the construction of composite or doublet Diamond simulants. The two materials are used for the bottom portion (pavilion) of the stone, and in the case of strontium titanate, a much harder material—usually colorless synthetic Spinel or Sapphire—is used for the top half (crown). In glass doublets, the top portion is made of almandine Garnet; it is usually a very thin slice which does not modify the stone's overall body color. There have even been reports of diamond-on-diamond doublets, where a creative entrepreneur has used two small pieces of rough to create one larger stone. In Strontium Titanate and Diamond-based doublets, an epoxy is used to adhere the two halves together. The epoxy may fluoresce under UV light, and there may be residue on the stone's exterior. The garnet top of a glass doublet is physically fused to its base, but in it and the other doublet types there are usually flattened air bubbles seen at the junction of the two halves. A join line is also readily visible whose position is variable; it may be above or below the girdle, sometimes at an angle, but rarely along the girdle itself. 
The most recent composite simulant involves combining a CZ core with an outer coating of laboratory created amorphous diamond. The concept effectively mimics the structure of a cultured pearl (which combines a core bead with an outer layer of pearl coating), only done for the diamond market.
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So again, get to know your jeweler or gemologist and don't be fooled by unknown vendors, salespersons and especially eBay and other auction sellers without having a chance to check-out the item completly yourself, with return rights.
.
.
Natural minerals that (when cut) optically resemble white diamonds are rare, because the trace impurities usually present
in natural minerals tend to impart color. The earliest simulants of diamond were colorless quartz, topaz and beryl (goshenite); they are all common minerals with above-average hardness (7–8), but all have low RIs and correspondingly low dispersions. Well-formed quartz crystals are sometimes offered as "diamonds," a popular example being the so-called "Herkimer diamond". Topaz's SG (3.50–3.57) also falls within the range of diamond..
From a historical perspective, the most notable natural simulant of diamond is Zircon.
It is also fairly hard (7.5), but more importantly shows perceptible fire when cut, due to its high dispersion of 0.039. Colorless zircon has been mined in Sri Lanke for over 2,000 years; prior to the advent of modern mineralogy, colorless zircon was thought to be an inferior form of diamond. It was called "Matara diamond" after its source location. It is still encountered as a diamond simulant, but differentiation is easy due to zircon's anisotropy and strong birefringence (0.059). It is also notoriously brittle and often shows wear on the girdle and facet edges. Much less common than colorless zircon is colorless scheelite. Its dispersion (0.026) is also high enough to mimic diamond, but although it is highly lustrous its hardness is much too low (4.5–5.5) to maintain a good polish. It is also anisotropic and fairly dense (SG 5.9–6.1). Synthetic scheelite produced via the Czochralski process is available, but it has never been widely used as a diamond simulant. Due to the scarcity of natural gem-quality scheelite, synthetic scheelite is much more likely to simulate it than diamond. A similar case is the orthorhombic carbonate cerussite, which is so fragile (very brittle with four directions of good cleavage) and soft (hardness 3.5) that it is never seen set in jewelry, and only occasionally seen in gem collections because it is so difficult to cut. Cerussite gems have an adamantine luster, high RI (1.804–2.078), and high dispersion (0.051), making them attractive and valued collector's pieces. Aside from softness, they are easily distinguished by cerussite's high density (SG 6.51) and anisotropy with extreme birefringence (0.271)..
Desert Diamonds are also known as Saudi Diamonds, or sometimes Qaysumah Diamonds.
They are natural, semi-precious stones from the same micro-crystalline family as Amethyst, smoky Topaz and Citrine. These unique, high-grade stones have physical properties that produce the appearance of the expensive carbon-based diamonds! DESERT DIAMONDS are FOREVER, just like their carbon diamonds cousin. They are guaranteed not to discolor or break with age. This is the huge advantage of owning Dessert Diamonds vs. cubic zirconia. Compared with the carbon-based stones, which have a hardness factor of 10 on the Mohs scale, the Desert Diamond are only rated at a mere 7.0 on the Moh's Scale!.
Due to their rarity fancy-colored diamonds are also imitated, and zircon can serve this purpose too. Applying heat treatment to brown zircon can create several bright colors: these are most commonly sky-blue, golden yellow, and red.
Blue zircon is very popular, but it is not necessarily color stable; prolonged exposure to ultraviolet light (including the UV component in sunlight) tends to bleach the stone. Heat treatment also imparts greater brittleness to zircon and characteristic inclusions. Another fragile candidate mineral is Sphaerite (zinc blende). Gem-quality material is usually a strong yellow to honey brown, orange, red, or green; its very high RI (2.37) and dispersion (0.156) make for an extremely lustrous and fiery gem, and it is also isotropic. But here again, its low hardness (2.5–4) and perfect dodecahedral cleavage preclude sphalerite's wide use in jewelry. 
Two calcium-rich members of the Garnet group fare much better: these are grossularite (usually brownish orange, rarely colorless, yellow, green, or pink) and andradite. The latter is the rarest and most costly of the Garnets, with three of its varieties—topazolite (yellow), melanite (black), and demantoid (green)—sometimes seen in jewelry. 
Demantoid (literally "diamond-like") especially has been prized as a gemstone since its discovery in the Ural Mountains in 1868; it is a noted feature of antique Russian and Art Nouveau jewelry. Titanite or Sphene is also seen in antique jewelry; it is typically some shade of chartreuse and has a luster, RI (1.885–2.050), and dispersion (0.051) high enough to be mistaken for diamond, yet it is anisotropic (a high birefringence of 0.105–0.135) and soft (hardness 5.5). Discovered in the 1960s, the rich green tsavorite variety of grossular is also very popular. Both grossular and andradite are isotropic and have relatively high RIs (ca. 1.74 and 1.89, respectively) and high dispersions (0.027 and 0.057), with demantoid's exceeding diamond. However, both have a low hardness (6.5–7.5) and invariably possess inclusions atypical of diamond—the byssolite "horsetails" seen in demantoid are one striking example. Furthermore, most are very small, typically under 0.5 carats (100 mg) in weight. Their lusters range from vitreous to subadamantine, to almost metallic in the usually opaque melanite, which has been used to simulate black diamond. Some natural spinel is also a deep black and could serve this same purpose..
Because Strontium Titanate and glass are too soft to survive use as a ring stone,
they have been used in the construction of composite or doublet Diamond simulants. The two materials are used for the bottom portion (pavilion) of the stone, and in the case of strontium titanate, a much harder material—usually colorless synthetic Spinel or Sapphire—is used for the top half (crown). In glass doublets, the top portion is made of almandine Garnet; it is usually a very thin slice which does not modify the stone's overall body color. There have even been reports of diamond-on-diamond doublets, where a creative entrepreneur has used two small pieces of rough to create one larger stone. In Strontium Titanate and Diamond-based doublets, an epoxy is used to adhere the two halves together. The epoxy may fluoresce under UV light, and there may be residue on the stone's exterior. The garnet top of a glass doublet is physically fused to its base, but in it and the other doublet types there are usually flattened air bubbles seen at the junction of the two halves. A join line is also readily visible whose position is variable; it may be above or below the girdle, sometimes at an angle, but rarely along the girdle itself. 
The most recent composite simulant involves combining a CZ core with an outer coating of laboratory created amorphous diamond. The concept effectively mimics the structure of a cultured pearl (which combines a core bead with an outer layer of pearl coating), only done for the diamond market..
So again, get to know your jeweler or gemologist and don't be fooled by unknown vendors, salespersons and especially eBay and other auction sellers without having a chance to check-out the item completly yourself, with return rights.
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