Diamond simulant

Desired and differential properties

See also: Material properties of 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.

Durability and density

The facets (described as adamantine) which are perfectly flat, and its crisp, sharp facet edges. For a diamond simulant to be effective, it must be very hard relative to most gems. Most simulants fall far short of diamond's hardness, so they can be separated from diamond by their external flaws and poor polish.

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 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.

Thermal and electrical

Diamond is an extremely effective moissanite, which has a thermal conductivity similar to diamond: older probes can be fooled by moissanite, but newer testers are sophisticated enough to differentiate the two materials.^{[citation needed]}

A diamond's electrical conductance is only relevant to blue or gray-blue stones, because the interstitial semiconductors. Thus a suspected blue diamond can be affirmed if it completes an electric circuit successfully.

Artificial simulants

Diamond has been imitated by artificial materials for hundreds of years: advances in technology have seen the development of increasingly better simulants with properties ever nearer those of diamond. Although most of these simulants were characteristic of a certain time period, their large production volumes ensured that all continue to be encountered with varying frequency in jewelry of the present. Nearly all were first conceived for intended use in high technology, such as lasing mediums, varistors, and bubble memory. Due to their limited present supply, collectors may pay a premium for the older types.

Summary table

Diamond simulants and their gemological properties

Material	Formula	Refractive index(es) 589.3 nm	Dispersion 431 - 687 nm	Mohs' scale)	Density (g/cm3)	Thermal Cond.	State of the art
Diamond	C	2.417	0.044	10	3.52	Excellent	1476 –
Artificial Simulants:
Glasses	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 (see patents)	3.2	High	1998 –

The "refractive index(es)" column shows one refractive index for singly refractive substances, and a range for doubly refractive substances.

1700 onwards

The formulation of glasses using conchoidal fractures, and air bubbles or flow lines within the stone, these features make glass imitations easy to spot under only moderate magnification. In contemporary production it is more common for glass to be molded rather than cut into shape: in these stones the facets will be concave and facet edges rounded, and mold marks or seams may also be present. Glass has also been combined with other materials to produce composites.

1900–1947

The first boule crystal. The process is an economical one, with crystals of up to 9 centimeters (3.5 inches) in diameter grown. Boules grown via the modern Czochralski process may weigh several kilograms.

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.

1947–1970

The first of the optically "improved" simulants was synthetic opal-like in their display of prismatic colors. Synthetic rutile is also doubly refractive: although some stones are cut with the table perpendicular to the optic axis to hide this property, merely tilting the stone will reveal the doubled back facets.

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 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, 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.

1970–1976

From about 1970 strontium titanate began to be replaced by a new class of diamond imitations: the "synthetic rare earth elements. They are the only diamond simulants (aside from rhinestones) with no known natural counterparts: gemologically they are best termed artificial rather than synthetic, because the latter term is reserved for human-made materials that can also be found in nature.

Although a number of artificial garnets were successfully grown, only two became important as diamond simulants. The first was 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 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 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.

1976 to present

skull crucible (in reference to either the shape of the crucible or of the crystals grown).

At 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 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 simulated moissanite (SiC; synthetic silicon carbide). Simulated moissanite is superior to cubic zirconia in two ways: its hardness (8.5-9.25, see relative patents) 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, simulated moissanite is strongly birefringent. This manifests as the same "drunken vision" effect seen in synthetic rutile, although to a lesser degree. All simulated 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 simulated 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 simulated moissanite due to its birefringence. Like synthetic rutile, current simulated 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 simulated moissanite is produced by only one company, Charles & Colvard. Its limited availability makes simulated moissanite about 120 times more expensive than cubic zirconia.

Natural simulants

Natural Herkimer diamonds" mined in Herkimer County, New York. 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 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 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 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.

Composites

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. Brought to market under the 'Asha' brand name, the finished simulant provides a more lustrous and diamond-like look than plain CZ due to its usage of amorphous diamond.

See also

• Diamond enhancement
• Diamond color
• Diamond cut
• Diamond clarity
• Material properties of diamond
• Synthetic diamond

References

• Hall, Cally. (1994). Gemstones, p. 63, 70, 121. Eyewitness Handbooks; Kyodo Printing Co., Singapore. ISBN 0-7737-2762-0
• Nassau, Kurt. (1980). Gems made by man, pp. 203–241. Gemological Institute of America; Santa Monica, California. ISBN 0-87311-016-1
• O'Donoghue, Michael, and Joyner, Louise. (2003). Identification of gemstones, pp. 12–19. Butterworth-Heinemann, Great Britain. ISBN 0-7506-5512-7
• Pagel-Theisen, Verena. (2001). Diamond grading ABC: The manual (9th ed.), pp. 298–313. Rubin & Son n.v.; Antwerp, Belgium. ISBN 3-9800434-6-0
• Schadt, H. (1996). Goldsmith's art: 5000 years of jewelry and hollowware, p. 141. Arnoldsche Art Publisher; Stuttgard, New York. ISBN 3-925369-54-6
• Webster, Robert, and Read, Peter G. (Ed.) (2000). Gems: Their sources, descriptions and identification (5th ed.), pp. 65–71. Butterworth-Heinemann, Great Britain. ISBN 0-7506-1674-1