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
active laser mediums,
varistors, and
bubble memory. Due to their limited present supply, collectors may pay a premium for the older types.
Summary table 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
flint glass using
lead,
alumina, and
thallium to increase RI and dispersion began in the late
Baroque period. Flint glass is fashioned into brilliants, and when freshly cut they can be surprisingly effective diamond simulants. Known as rhinestones, pastes, or strass, glass simulants are a common feature of
antique jewelry; in such cases, rhinestones can be valuable historical artifacts in their own right. The great softness (below hardness 6) imparted by the lead means a rhinestone's facet edges and faces will quickly become rounded and scratched. Together with
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
crystalline artificial diamond simulants were synthetic white
sapphire (
Al2O3, pure corundum) and
spinel (MgO·Al2O3, pure
magnesium aluminium
oxide). Both have been synthesized in large quantities since the first decade of the 20th century via the
Verneuil or flame-fusion process, although spinel was not in wide use until the 1920s. The Verneuil process involves an inverted
oxyhydrogen blowpipe, with purified feed powder mixed with
oxygen that is carefully fed through the blowpipe. The feed powder falls through the oxy-hydrogen flame, melts, and lands on a rotating and slowly descending pedestal below. The height of the pedestal is constantly adjusted to keep its top at the optimal position below the flame, and over a number of hours the molten powder cools and crystallizes to form a single pedunculated pear or
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; however, 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 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
Radiant.
1947–1970 The first of the optically "improved" simulants was synthetic rutile (TiO2, pure
titanium oxide). Introduced in 1947–48, synthetic rutile possesses plenty of life when cut—perhaps too much life for a diamond simulant. Synthetic rutile's RI and dispersion (2.8 and 0.33) are so much higher than diamond that the resultant brilliants look almost
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 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
NL 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. 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
garnets". These are not true garnets in the usual sense because they are oxides rather than
silicates, but they do share natural garnet's
crystal structure (both are cubic and therefore isotropic) and the general formula A3B2C3O12. While in natural garnets C is always
silicon, and A and B may be one of several common
elements, most synthetic garnets are composed of uncommon 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
yttrium aluminium garnet (
YAG; Y3Al5O12) in the late 1960s. It was (and still is) produced by 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 near 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) in length, and weighs 9,000 carats (1.75 kg). YAG 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
Shelby Gem Factory of Michigan,
Litton Systems,
Allied Chemical,
Raytheon, and Union Carbide; annual global production peaked at 40 million carats (8000 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 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.
Since 1976 Cubic zirconia or CZ (ZrO2;
zirconium dioxide—not to be confused with
zircon, a
zirconium silicate) quickly dominated the diamond simulant market following its introduction in 1976, and it remains the most gemologically and economically important simulant. CZ had been synthesized since 1930 but only in
ceramic form: the growth of single-crystal CZ would require an approach radically different from those used for previous simulants due to zirconia's extremely high melting point (2750 °C), unsustainable by any crucible. The solution found involved a network of water-filled copper pipes and
radio-frequency induction heating coils; the latter to heat the zirconia feed powder, and the former to cool the exterior and maintain a retaining "skin" under 1 millimeter thick. CZ was thus grown in a crucible of itself, a technique called
cold crucible (in reference to the cooling pipes) or
skull crucible (in reference to either the shape of the crucible or of the crystals grown). 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(III) oxide or
calcium oxide. The skull crucible technique was first developed in 1960s
France, but 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 most popular simulant of diamond. Its optical and physical constants are however variable, owing to the different stabilizers used by different producers. 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. Cubic zirconia can be coated with
diamond-like carbon to improve its durability, but will still be detected as CZ by a thermal probe. CZ had virtually no competition until the 1998 introduction of
moissanite (SiC;
silicon carbide). Moissanite is superior to cubic zirconia in three ways: its
hardness (8.5–9.25); low
specific gravity (3.2);
refractive index of 2.65, the highest known. The former property results in facets that are 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 perpendicular 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 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. == Natural simulants ==