The high price of gem-grade diamonds has created a large demand for materials with similar gemological characteristics, known as diamond simulants or imitations. Simulants are distinct from synthetic diamond, which unlike simulants is actual diamond, and therefore has the same material properties as natural diamond. Enhanced diamonds are also excluded from this definition. A diamond simulant may be artificial, natural, or in some cases a combination thereof. While their material properties depart markedly from those of diamond, simulants have certain desired characteristics—such as dispersion and hardness—which lend themselves to imitation. Trained gemologists with appropriate equipment are able to distinguish natural and synthetic diamonds from all diamond simulants, primarily by visual inspection.
The most common diamond simulants are high-leaded glass (i.e., rhinestones) and cubic zirconia (CZ), both artificial materials. A number of other artificial materials, such as strontium titanate and synthetic rutile have been developed since the mid 1950s, but these are no longer in common use. Introduced at the end of the 20th century, the lab grown product moissanite has gained popularity as an alternative to diamond.
In order to be considered for use as a diamond simulant, a material must possess certain diamond-like properties. The most advanced artificial simulants have properties which closely approach diamond, but all simulants have one or more features that clearly and (for those familiar with diamond) easily differentiate them from diamond. To a gemologist, the most important of differential properties are those that foster non-destructive testing, and most of these are visual in nature. Non-destructive testing is preferred because most suspected diamonds are already cut into gemstones and set in jewelry, and if a destructive test (which mostly relies on the relative fragility and softness of non-diamonds) fails it may damage the simulant—this is not an acceptable outcome for most jewelry owners, as even if a stone is not a diamond it may still be of value.
Following are some of the properties by which diamond and its simulants can be compared and contrasted.
In the recent past, the so-called "window pane test" was commonly thought to be an assured method of identifying diamond. It is a potentially destructive test wherein a suspect diamond gemstone is scraped against a pane of glass, with a positive result being a scratch on the glass and none on the gemstone. The use of hardness points and scratch plates made of corundum (hardness 9) are also used in place of glass. Hardness tests are inadvisable for three reasons: glass is fairly soft (typically 6 or below) and can be scratched by a large number of materials (including many simulants); diamond has four directions of perfect and easy cleavage (planes of structural weakness along which the diamond could split) which could be triggered by the testing process; and many diamond-like gemstones (including older simulants) are valuable in their own right.
The specific gravity (SG) or density of a gem diamond is fairly constant at 3.52. Most simulants are far above or slightly below this value, which can make them easy to identify if unset. High-density liquids such as diiodomethane can be used for this purpose, but they are all highly toxic so are usually avoided. A more practical method is to compare the expected size and weight of a suspect diamond to its measured parameters: for example, a cubic zirconia (SG 5.6–6) will be 1.7 times the expected weight of an equivalently sized diamond.
Perhaps equally as important is optic character. Diamond and other cubic (and also amorphous) materials are isotropic, meaning light entering a stone behaves the same way regardless of direction. Conversely, most minerals are anisotropic which produces birefringence or double refraction of light entering the material in all directions other than an optic axis (a direction of single refraction in a doubly refractive material). Under low magnification, this birefringence is usually detectable as a visual doubling of a cut gemstone's rear facets or internal flaws. An effective diamond simulant should therefore be isotropic.
Under longwave (365 nm) ultraviolet light, diamond may fluoresce a blue, yellow, green, mauve, or red of varying intensity. The most common fluorescence is blue, and such stones may also phosphoresce yellow—this is thought to be a unique combination among gemstones. There is usually little if any response to shortwave ultraviolet, in contrast to many diamond simulants. Similarly, because most diamond simulants are artificial they tend to have uniform properties: in a multi-stone diamond ring, one would expect the individual diamonds to fluoresce differently (in different colors and intensities, with some likely to be inert). If all the stones fluoresce in an identical manner, they are unlikely to be diamond.
Most "colorless" diamonds are actually tinted yellow or brown to some degree, whereas artificial simulants are usually completely colorless—the equivalent of a perfect "D" in diamond color terminology. This "too good to be true" factor is important to consider; colored diamond simulants meant to imitate fancy diamonds are more difficult to spot in this regard, but the simulants' colors rarely approximate. In most diamonds (even colorless ones) a characteristic absorption spectrum can be seen (via a direct-vision spectroscope), consisting of a fine line at 415 nm. The dopants used to impart color in artificial simulants may be detectable as a complex rare earth absorption spectrum, which is never seen in diamond.
Also present in most diamonds are certain internal and external flaws or inclusions, the most common of which are fractures and solid foreign crystals. Artificial simulants are usually internally flawless, and any flaws that are present are characteristic of the manufacturing process. The inclusions seen in natural simulants will often be unlike those ever seen in diamond, most notably liquid "feather" inclusions. The diamond cutting process will often leave portions of the original crystal's surface intact. These are termed naturals and are usually on the girdle of the stone; they take the form of triangular, rectangular, or square pits (etch marks) and are seen only in diamond.
A diamond's electrical conductance is only relevant to blue or gray-blue stones, because the interstitial boron responsible for their color also makes them semiconductors. Thus a suspected blue diamond can be affirmed if it completes an electric circuit successfully.
431 - 687 nm
| State of|
|Glasses||Silica with Pb, Al, &/or Tl||~ 1.6||> 0.020||< 6||2.4 – 4.2||Poor||1700 –|
|White Sapphire||Al2O3||1.762 – 1.770||0.018||9||3.97||Poor||1900 – 1947|
|Spinel||MgO·Al2O3||1.727||0.020||8||~ 3.6||Poor||1920 – 1947|
|Rutile||TiO2||2.62 – 2.9||0.33||~ 6||4.25||Poor||1947 – 1955|
|Strontium titanate||SrTiO3||2.41||0.19||5.5||5.13||Poor||1955 – 1970|
|YAG||Y3Al5O12||1.83||0.028||8.25||4.55 – 4.65||Poor||1970 – 1975|
|GGG||Gd3Ga5O12||1.97||0.045||7||7.02||Poor||1973 – 1975|
|Cubic Zirconia||ZrO2(+ rare earths)||~ 2.2||~ 0.06||~ 8.3||~ 5.7||Poor||1976 –|
|Moissanite||SiC||2.648 – 2.691||0.104||8.5-9.25||3.2||High||1998 –|
The "refractive index(es)" column shows one refractive index for singly refractive substances, and a range for doubly refractive substances.
Synthetic sapphire and spinel are durable materials (hardness 9 and 8) that take a good polish, but due to their much lower RI when compared to diamond (1.762–1.770 for sapphire, 1.727 for spinel) they are "lifeless" when cut. (Synthetic sapphire is also anisotropic, making it even easier to spot.) Their low RIs also mean a much lower dispersion (0.018 and 0.020), so even when cut into brilliants they lack the fire of diamond. Nevertheless synthetic spinel and sapphire were popular diamond simulants from the 1920s up until the late 1940s, when newer and better simulants began to appear. Both have also been combined with other materials to create composites. Commercial names once used for synthetic sapphire include Diamondette, Diamondite, Jourado Diamond', and Thrilliant. Names for synthetic spinel included Corundolite, Lustergem, Magalux, and Radient.
The continued success of synthetic rutile was also hampered by the material's inescapable yellow tint, which producers were never able to remedy. However, synthetic rutile in a range of different colors, including blues and reds, were produced using various metal oxide dopants. These and the near-white stones were extremely popular if unreal stones. Synthetic rutile is also fairly soft (hardness ~6) and brittle, and therefore wears poorly. It is synthesized via a modification of the Verneuil process, which uses a third oxygen pipe to create a tricone burner—this is necessary to produce a single crystal, due to the much higher oxygen losses involved in the oxidation of titanium. The technique was invented by Charles H. Moore, Jr. at the South Amboy, New Jersey-based National Lead Company (later N. L. Industries). National Lead and Union Carbide were the primary producers of synthetic rutile, and peak annual production reached 750,000 carats (150 kg). Some of the many commercial names applied to synthetic rutile include: Astryl, Diamothyst, Gava or Java Gem, Meredith, Miridis, Rainbow Diamond, Rainbow Magic Diamond, Rutania, Titangem, Titania, and Ultamite.
National Lead was also where research into the synthesis of another titanium compound, strontium titanate (SrTiO3, pure tausonite), was conducted. Research was done during the late 1940s and early 1950s by Leon Merker and Langtry E. Lynd, who also used a tricone modification of the Verneuil process. Upon its commercial introduction in 1955, strontium titanate quickly replaced synthetic rutile as the most popular diamond simulant. This was due not only to strontium titanate's novelty, but to its superior optics: its RI (2.41) is very close to that of diamond, while its dispersion (0.19), although also very high, was a significant improvement over synthetic rutile's psychedelic display. Perhaps most importantly was the complete lack of yellow tint that so plagued synthetic rutile. Dopants were also used to give synthetic titanate a variety of colors, including yellow, orange to red, blue, and black. The material is also isotropic like diamond, meaning there is no distracting doubling of facets as seen in synthetic rutile.
Strontium titanate's only major drawback (if one excludes excess fire) is fragility. It is both softer (hardness 5.5) and more brittle than synthetic rutile—for this reason, strontium titanate was also combined with more durable materials to create composites. It was otherwise the best simulant around at the time, and at its peak annual production was 1.5 million carats (300 kg). Due to patent coverage all US production was by National Lead, while large amounts were produced overseas by Nakazumi Company of Japan. Commercial names for strontium titanate included Brilliante, Diagem, Diamontina, Fabulite, and Marvelite.
Although a number of artificial garnets were successfully grown, only two became important as diamond simulants. The first was yttrium aluminium garnet (YAG; Y3Al5O12) in the late 1960s. It was (and still is) produced via the Czochralski or crystal-pulling process, which involves growth from the melt. An iridium crucible surrounded by an inert atmosphere is used, wherein yttrium oxide and aluminium oxide are melted and mixed together at a carefully controlled temperature of ca. 1980°C. A small seed crystal is attached to a rod which is lowered over the crucible until the crystal contacts the surface of the melted mixture. The seed crystal acts as a site of nucleation; the temperature is kept steady at a point where the surface of the mixture is just below the melting point. The rod is slowly and continuously rotated and retracted, and the pulled mixture crystallizes as it exits the crucible, forming a single crystal in the form of a cylindrical boule. The crystal's purity is extremely high, and it typically measures 5 cm (2 inches) in diameter and 20 cm (8 inches) long, and weighs 9,000 carats (1.75 kg).
YAG's hardness (8.25) and lack of brittleness were great improvements over strontium titanate, and although its RI (1.83) and dispersion (0.028) were fairly low, they were enough to give brilliant-cut YAGs perceptible fire and good brilliance (although still much lower than diamond). A number of different colors were also produced with the addition of dopants, including yellow, red, and a vivid green which was used to imitate emerald. Major producers included ICT, INC. of Michigan, Litton Systems, Allied Chemical, Raytheon, and Union Carbide; annual global production peaked at 40 million carats (8,000 kg) in 1972, but fell sharply thereafter. Commercial names for YAG included Diamonair, Diamonique, Gemonair, Replique, and Triamond.
While market saturation was one reason for the fall in YAG production levels, another was the recent introduction of the other artificial garnet important as a diamond simulant, gadolinium gallium garnet (GGG; Gd3Ga5O12). Produced in much the same manner as YAG (but with a lower melting point of 1750°C), GGG had an RI (1.97) close to, and a dispersion (0.045) nearly identical to diamond. GGG was also hard enough (hardness 7) and tough enough to be an effective gemstone, but its ingredients were also much more expensive than YAG's. Equally hindering was GGG's tendency to turn a dark brown upon exposure to sunlight or other ultraviolet source: this was due to the fact that most GGG gems were fashioned from impure material that was rejected for technological use. The SG of GGG (7.02) is also the highest of all diamond simulants and amongst the highest of all gemstones, which makes loose GGG gems easy to spot by comparing their dimensions with their expected and actual weights. Relative to its predecessors, GGG was never produced in significant quantities; it became more or less unheard of by the close of the 1970s. Commercial names for GGG included Diamonique II and Galliant.
At standard pressure zirconium oxide would normally crystallize in the monoclinic rather than cubic crystal system: for cubic crystals to grow, a stabilizer must be used. This is usually yttrium or calcium. The skull crucible technique was first developed in 1960s France, but it was perfected in the early 1970s by Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow. By 1980 annual global production had reached 50 million carats (10,000 kg).
The hardness (8–8.5), RI (2.15–2.18, isotropic), dispersion (0.058–0.066), and low material cost make CZ the best and most popular simulant of diamond. Its optical and physical constants are however variable, owing to the different stabilizers used by different producers. It is important to realize that CZ is not a compound. There are many formulations of stabilized cubic zirconia. These variations change the physical and optical properties markedly. While the visual likeness of CZ is close enough to diamond to fool most who do not handle diamond regularly, CZ will usually give certain clues. For example: it is somewhat brittle and is soft enough to possess scratches after normal use in jewelry; it is usually internally flawless and completely colorless (whereas most diamonds have some internal imperfections and a yellow tint); its SG (5.6–6) is high; and its reaction under ultraviolet light is a distinctive beige. Most jewelers will use a thermal probe to test all suspected CZs, a test which relies on diamond's superlative thermal conductivity (CZ, like almost all other diamond simulants, is a thermal insulator). CZ is made in a number of different colors meant to imitate fancy diamonds (e.g., yellow to golden brown, orange, red to pink, green, and opaque black), but most of these do not approximate the real thing. Some CZs have been given a coating of diamond-like carbon in an effort to improve their durability, but this does not fool a thermal probe.
CZ had virtually no competition until the 1998 introduction of moissanite (SiC; silicon carbide). Moissanite is superior to cubic zirconia in two ways: its hardness (8.5-9.25) and low SG (3.2). The former property results in facets that are as sometimes as crisp as a diamond's, while the latter property makes simulated moissanite somewhat harder to spot when unset (although still disparate enough to detect). However, unlike diamond and cubic zirconia, moissanite is strongly birefringent. This manifests as the same "drunken vision" effect seen in synthetic rutile, although to a lesser degree. All moissanite is cut with the table perpendicular to the optic axis in order to hide this property from above, but when viewed under magnification at only a slight tilt the doubling of facets (and any inclusions) is readily apparent.
The inclusions seen in moissanite are also characteristic: most will have fine, white, subparallel growth tubes or needles oriented perpedicular to the stone's table. It is conceivable that these growth tubes could be mistaken for laser drill holes that are sometimes seen in diamond (see diamond enhancement), but the tubes will be noticeably doubled in moissanite due to its birefringence. Like synthetic rutile, current moissanite production is also plagued by an as of yet inescapable tint, which is usually a brownish green. A limited range of fancy colors have been produced as well, the two most common being blue and green. Jewel-quality moissanite is produced by only one company, Charles & Colvard. Its limited availability makes moissanite about 120 times more expensive than cubic zirconia.
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 Lanka 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).
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 sphalerite (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 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.
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.