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Simulants vs. Synthetics: A concise guide to manmade gemstones

Written by davidelevi

Definitions of 'Synthetic' and 'Simulant'

Synthetic

A synthetic material is something that is created by man through chemical-physical processes. This may be something that also exists naturally (e.g. sapphire or ruby), or it may be a unique material without a natural equivalent (e.g. CZ or YAG). In this latter case, it is better termed 'artificial'.

Synthetic diamond means true diamond - pure carbon, crystallised in isotropic 3D form - created through a man-controlled process. Other appropriate terms for 'synthetic' include man-made, laboratory-grown, cultured, manufactured or created. The use of the denomination 'synthetic diamond' for any other material is unacceptable and should be denounced as fraud.

Simulant

A simulant is a material that imitates another one - that is, it behaves like the imitated material in some important respects, but it is not the same substance. For diamond and gemstones in general, the imitation usually tries to look as close to the original in terms of:

  • Color, transparency and luster
  • Refraction and dispersion of light
  • Hardness, toughness and polish

Diamond simulants may be natural (e.g. white zircon, quartz) or synthetic (e.g. glass, synthetic rutile) and are generally fairly easy to distinguish from diamond - natural or synthetic.

Other appropriate terms for 'simulant' include imitation and faux. The labels synthetic, man-made, laboratory-grown, cultured, manufactured or created - while not necessarily incorrect - are not complete descriptions of a simulant unless used together with the term 'simulant'.

Synthetic diamond production methods

The history of synthetic gem-quality diamonds goes back to the early 1970s, when General Electric produced the first synthetic diamond crystals large enough to be used for jewellery purposes. As of today, there are two production methods for synthetic diamonds: the High Pressure High Temperature (HPHT) process and the Carbon Vapour Deposition (CVD) process:

HPHT

The HPHT process is basically an attempt to replicate the natural process through which a diamond is created naturally. Graphite or diamond 'powder' is put into a high pressure (50,000 atmospheres) high temperature (1500 C) environment, in the presence of a metal solvent (iron or nickel) and one or several small diamond crystals to act as seeds. This results in diamonds that can grow as large as 10 carats, generally brown or yellow in colour due to the presence of nitrogen. Other colours, such as blue and colourless are achievable through the addition of suitable dopants and the elimination of nitrogen, although this results in much slower and more irregular growth of the diamond crystals.

CVD

In the CVD process, graphite is vaporised in a vacuum chamber where there is a diamond crystal. The carbon vapor deposits on the seed crystal and grows its size. Diamonds produced through this method are very pure and free of distortions - and thus colourless, but the crystals are small (below 1 ct of cut gem) and may have graphite inclusions.

Both HPHT and CVD diamonds can be irradiated to create red/pink or green stones.

Although the production of synthetic diamonds is many times that of natural diamonds, the vast majority of synthetic (and natural!) diamond production is of industrial quality for use as abrasives and surface finishes, as well as in microelectronics. Only few producers of gem-quality synthetic diamonds exist - at the time of writing, Apollo is the only manufacturer using the CVD method, while Chatham, D.Nea, Gemesis and Takara use the HPHT method.

Distinguishing a synthetic from a natural diamond is not an easy task, and normally cannot be accomplished outside a well equipped gemological laboratory, although the most common types of synthetics can be identified more easily. HPHT stones sometimes show traces of the metal solvent which are a tell tale sign of their origin, but these are rarely large enough to be seen with a loupe or an ordinary microscope. Synthetic diamonds show distinct UV and IR absorption behaviour, and infrared Raman spectrography is the only reliable method to distinguish synthetic from natural.

Common Diamond Simulants

The history of diamond simulants is about as old as the use of cut diamonds - in fact, it is probably older, since some stones that are used as diamond simulants are much more common and easier to work with than diamonds, and thus were used earlier than true diamond in the making of jewellery. Diamond simulants can be natural or synthetic, although many of the natural simulants are also produced through synthesis.

Natural simulants

Natural colourless stones used as diamond simulants include beryl (goshenite), corundum (white sapphire), quartz, spinel, topaz, and zircon. All of these also exist in synthetic form, and all except zircon are easily distinguished from diamond because of ther lower Refractive Index (RI) and dispersion. Zircon has RI and dispersion more similar to diamond, but it is doubly refringent and much softer than diamond.

Early synthetics

Historically, the first synthetic simulants (as early as the 17th century) were various forms of glass. The "simulation" was aided by the custom of setting stones in closed, often foil-backed setting, and by the poor cut and polish of early diamonds. Well cut lead glass can be a surprisingly effective simulant (think Swarovski) due to its reasonable dispersion, but it is soft and wears easily, and it has a RI much lower than diamond.

Synthetic corundum and spinel go back to the early 1900s, and are widely spread in colourless, red and blue colours to imitate respectively diamond, ruby and sapphire. Although they are quite hard and lustrous, thus durable and taking a good polish, they aren't realistic diamond simulants because of low RI and dispersion. Of course, they are perfect simulants for natural sapphire and spinel - but they are still relatively easily separated from natural stones, in most cases through simple loupe examination.

1950s synthetics

With the growing diffusion of wealth during the 1950s, demand for better diamond simulants led to the introduction of synthetic rutile and strontium titanate.

Synthetic rutile (titanium dioxide, TiO2) has one of the highest RI and dispersion of any material, way above that of diamond, and - apart from its hardness - it is easily distinguished from diamond because of this: it has a very artificial look with lots of coloured fire; in addition, it has a yellow body colour that is not easily disguised. Synthetic rutile is perfectly crystalline, unlike naturally occurring rutile that has a metallic sheen and generally appears opaque or translucent because of the crystal size.

Strontium titanate (SrTiO3) has optical characteristics close to diamond - although its dispersion is still too high, it is colourless and it has an RI that is an excellent match to diamond. However, it is far too soft (Mohs 5.5) and brittle for use in jewellery, particularly rings. The natural form of SrTiO3 (tausonite) was discovered in 1982, long after the heyday of SrTiO3 as a diamond simulant.

Composite stones (doublets) with a sapphire or spinel crown and a rutile or strontium titanate pavillion also exist, to join the desirable optical properties of rutile and SrTiO3 with the durability and hardness of sapphire and spinel.

SrTiO3 and rutile are still produced for industrial purposes, but their use as gem simulants has declined to the point that finding newly cut examples is now rather difficult.

The 1970s - artificial simulants

The greater availability of 'rare earth' materials in the 1970s following advances in electronics and nuclear technology made possible the creation of totally new materials that could be used - among other things - as gem simulants. The two of greater interest in jewellery are Yttrium Aluminium Garnet (YAG; introduced 1970) and Gadolinium Gallium Garnet (GGG; introduced 1973). These are characterised by relatively good hardness (Mohs 7-8), a wide range of body colouring (colourless, blue, green, red, orange, purple, yellow, brown) and - particularly for GGG - high RI and dispersion.

All these characteristics make them fairly good simulants for coloured gemstones, and diamond-like optics are within reach for GGG; unfortunately, they are both (especially GGG) far more expensive than other simulants. After the introduction of cubic zirconia in 1976, both these materials became infrequently used for jewellery purposes, though they remain in industrial use for solid state lasers and other opto-electronic components.

YAG is fairly easily differentiated from diamond because it tends to appear glassy and "lifeless", since it has a considerably lower RI and dispersion. GGG is much closer (though still less lively than the real thing), but both can be detected easily when loose because of their specific gravity: for a given size, they weigh 50% to 100% more than a diamond. They are also poor thermal conductors - like all the other simulants except moissanite.

Cubic zirconia (CZ or ZrO2) is also a largely artificial material - although it occurs naturally as a result of radioactivity on zircon crystals, the resulting crystals are microscopic and have no commercial use. The introduction on the market of CZ happened in 1976, and it was a near-instant success. Production in 1980 exceeded 1/5 of the total production of diamonds in weight (and about 90% of those are for industrial use!), and it has gone up since.

CZ is perfectly colourless, but it can be produced in a range of colours through appropriate dopants, although its high RI and dispersion make it a credible simulant only for diamond. It is cheap to produce, it is reasonably durable (Mohs 8.5) and it can be polished highly - although the cost of quality polishing and cutting becomes a limiting factor considering its typical retail price. It is easily separated from diamonds through its poor thermal conductivity (those "Diamond Tester" gadgets) and much higher specific gravity.

Simulants of the year 2000 and beyond

So, is CZ the ultimate diamond simulant? Well, some people think not quite. The last ten years have seen the introduction of two more diamond simulants: 'diamond coated' CZ, and moissanite.

'Diamond coated' CZ uses a CVD-like process to deposit a thin film of polycrystalline diamond on a cubic zirconia core. This has a number of advantages compared to a simple CZ:

  • Hardness increases to somewhere above Mohs 9 (the actual hardness of diamond is not a fixed Mohs 10 - it depends significantly on its crystalline form, with some forms being harder than single crystal diamonds and others being softer)
  • Loss of luster/shine due to dirt accumulating in CZ's porous surface is decreased, since diamond isn't porous
  • Dispersion - which in simple CZ is detectably higher than in diamond - is attenuated, thus creating a more realistic simulant
  • Costs don't increase significantly, and the coating treatment can be done in non-challenging environments (low pressure and temperature).

Moissanite is another substance occurring naturally in minimal amounts - for example in meteorites - that has interesting characteristics as a gem simulant. It is extremely hard, it has high RI and dispersion (too high compared to diamond; but rainbow sparkles are nice) and - apart from a not-very-nice greenish-brown body colour is quite an acceptable simulant. Unfortunately, it is rather costly because only one company (Charles & Colvard) currently produces gem-quality moissanite in any quantity.

Moissanite can be discriminated from diamond very easily because it conducts electricity (which diamond does not), and it is doubly refringent. However, since it is a good heat conductor, it fools the "older" generation of hand-held diamond testers.

The future may bring other materials and technologies to the field of simulants, but my personal feeling is that beating the CZ combination of looks, durability and low cost will be very hard. I think it more likely that the next step in diamond substitutes will be the widespread availability of much lower cost synthetics.