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Diamond simulant

2007 Schools Wikipedia Selection. Related subjects: Materials science

   Due to its low cost and close visual likeness to diamond, cubic
   zirconia has remained the most gemologically and economically important
   diamond simulant since 1976.
   Due to its low cost and close visual likeness to diamond, cubic
   zirconia has remained the most gemologically and economically important
   diamond simulant since 1976.

   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
   artificial product moissanite has gained popularity as a supposedly
   superior diamond simulant, although its much higher cost and limited
   production have kept it a relatively minor simulant.

Desired and differential properties

   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 Mohs scale of mineral hardness is a non-linear scale of common
   minerals' resistances to scratching. Diamond is at the top of this
   scale (hardness 10) as it is the hardest naturally occurring material
   known (the hardest substance known today is the man-made substance
   aggregated diamond nanorods). Since diamonds are unlikely to encounter
   substances that can scratch it, other than another diamond, diamond
   gemstones are typically free of scratches. Diamond's hardness also is
   visually evident (under the microscope or loupe) by its highly lustrous
   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 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 di-iodomethane 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.

Optics and colour

   Diamonds are usually cut into brilliants to bring out their brilliance,
   the amount of light reflected back to the viewer, and fire, the degree
   of colorful prismatic flashes seen. Both properties are strongly
   affected by the cut of the stone, but they are a function of diamond's
   high refractive index (RI; the degree to which incident light is bent
   upon entering the stone) of 2.417 (as measured by sodium light, 589.3
   nm) and high dispersion (the degree to which white light is split into
   its spectral colors within the stone) of 0.044, as measured by the
   sodium B and G line interval. Thus, if a diamond simulant's RI and
   dispersion are too low it will appear comparatively dull or "lifeless";
   if the RI and dispersion are too high, the effect will be considered
   unreal or even tacky. Very few simulants have closely approximating RI
   and dispersion, but even the close simulants can be separated by an
   experienced observer. Direct measurements of RI and dispersion are
   impractical (a standard gemological refractometer has an upper limit of
   about RI 1.81), but several companies have devised reflectivity meters
   to gauge a material's RI indirectly by measuring how well it reflects
   an infrared beam.

   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 colour
   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 colour
   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.

Thermal and electrical

   Diamond is an extremely effective thermal conductor and usually an
   electrical insulator. The former property is widely exploited in the
   use of an electronic thermal probe to separate diamonds from their
   imitations. These probes consist of a pair of battery-powered
   thermistors mounted in a fine copper tip. One thermistor functions as a
   heating device while the other measures the temperature of the copper
   tip: if the stone being tested is a diamond, it will conduct the tip's
   thermal energy rapidly enough to produce a measurable temperature drop.
   As most simulants are thermal insulators, the thermistor's heat will
   not be conducted. This test takes about 2–3 seconds. The only possible
   exception is 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.

   A diamond's electrical conductance is only relevant to blue or
   gray-blue stones, because the interstitial boron responsible for their
   colour also makes them 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

   CAPTION: Diamond simulants and their gemological properties

   Material Formula Refractive
   index(es)
   589.3 nm Dispersion
   431 - 687 nm Hardness
   (Mohs'
   scale) Density
   (g/cm^3) Thermal
   Cond. State of
   the art
   Diamond C 2.417 0.044 10 3.52 Excellent 1476 –
   Artificial Simulants:
   Glasses Silica with Pb, Al, &/or Tl ~ 1.6 > 0.020 < 6 2.4 – 4.2 Poor
   1700 –
   White Sapphire Al[2]O[3] 1.762 – 1.770 0.018 9 3.97 Poor 1900 – 1947
   Spinel MgO·Al[2]O[3] 1.727 0.020 8 ~ 3.6 Poor 1920 – 1947
   Rutile TiO[2] 2.62 – 2.9 0.33 ~ 6 4.25 Poor 1947 – 1955
   Strontium titanate SrTiO[3] 2.41 0.19 5.5 5.13 Poor 1955 – 1970
   YAG Y[3]Al[5]O[12] 1.83 0.028 8.25 4.55 – 4.65 Poor 1970 – 1975
   GGG Gd[3]Ga[5]O[12] 1.97 0.045 7 7.02 Poor 1973 – 1975
   Cubic Zirconia ZrO[2](+ rare earths) ~ 2.2 ~ 0.06 ~ 8.3 ~ 5.7 Poor 1976
   –
   Moissanite SiC 2.648 – 2.691 0.104 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.

1700 onwards

   The formulation of glasses using lead, alumina, and thallium to
   increase RI and dispersion began in the late Baroque period. These
   glasses are 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, and
   in such cases rhinestones can be valuable historical artifacts in their
   own right. The great softness (below hardnes 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 (Al[2]O[3], pure corundum) and spinel (MgO·Al[2]O[3], 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 in) 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 rutile
   (TiO[2], 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 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 (SrTiO[3], 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 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
   A[3]B[2]C[3]O[12]. 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; Y[3]Al[5]O[12]) 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;
   Gd[3]Ga[5]O[12]). 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.

1976 to present

   Cubic zirconia or CZ (ZrO[2]; zirconium oxide—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 zirconium'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 coils; the latter to heat the zirconium 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 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. 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
   synthetic moissanite (SiC; silicon carbide). Synthetic moissanite is
   superior to cubic zirconia in two ways: its great hardness (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
   moissanite somewhat harder to spot when unset (although still disparate
   enough to detect). Synthetic moissanite is also more resistant to heat
   than any other gemstone: so much so that it can be safely set directly
   into molten gold. However, its dispersion (0.104) is over twice that of
   diamond; it is also anisotropic with an RI of 2.648–2.691 and a high
   birefringence of 0.043. This manifests as the same "drunken vision"
   effect seen in synthetic rutile, although to a lesser degree. All
   synthetic 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 synthetic 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. Gem-quality synthetic moissanite is
   produced by only one company, Charles & Colvard. Its limited
   availability makes moissanite about 120 times more expensive than cubic
   zirconia.

   When synthetic moissanite was first introduced it made quite a stir:
   stories of widespread fraud were circulated by the press, with claims
   that synthetic moissanite was indistinguishable from diamond even by
   experts. The aforementioned properties clearly demonstrate this to be
   false: the only people fooled by synthetic moissanite were those who
   relied too heavily on thermal probes. This is because, like diamond,
   moissanite has a high thermal conductivity; probes manufactured before
   synthetic moissanite's introduction therefore registered synthetic
   moissanite as diamond. More sophisticated thermal probes are now able
   to differentiate moissanite from diamond, and Charles & Colvard also
   manufacture their own proprietary device which relies on moissanite's
   greater opacity to ultraviolet light.

Natural simulants

   Natural minerals that (when cut) optically resemble white diamonds are
   rare, because the trace impurities usually present in natural minerals
   tend to impart colour. 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 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 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 colour 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 grossular (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.

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 colour. 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.

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