Synthetic moissanite
| Synthetic Moissanite | |
|---|---|
| Chemical composition | Silicon Carbide - SiC |
| Crystal system | Hexagonal |
| Habit | Boule plates |
| Cleavage | Weak (good parting) |
| Fracture | Unknown |
| Hardness | 9.25 |
| Optic nature | Uniaxial + |
| Refractive index | 2.648 - 2.691 |
| Birefringence | 0.043 |
| Dispersion | High, 0.104 |
| Specific gravity | 3.22 |
| Lustre | Sub-adamantine |
| Pleochroism | Unknown |
Moissanite is a natural occurring mineral that is named after its discoverer, 1906 Nobel Prize (chemistry) winner Henri Moissan (1852-1907). As a natural source, this mineral is not suitable to be cut into gemstones as it occurs in too small grains and is very rare.
Moissanite (silicon carbide) was synthesized (1893) before it was discovered (1905) in nature.
Synthesizing moissanite has never been a real problem, however at first it was only synthesized for industrial purposes. This material, with a hardness of 9.25 on Moh's scale, is known by many bench jewelers, lapidarists and dentists as "carborundum" and is widely used as a sharpening tool for gravers and scrapers. As many who use carborundum can testify, this material is not very attractive as a gemstone.
Around 1987 Cree Research found a method to produce gemstone quality synthetic moissanite, but it was not until 1993 that a near-colorless crystal was obtained. The technique was patented in 1998 and the synthetics are created with the sublimation method. Today this gemstone synthetic is marketed as Charles & Colvard created Moissanite™.
Although a nice synthetic on its own, this material is used mainly as a diamond simulant. Since the arrival of synthetic cubic zirconia in 1976, this is the most convincing diamond simulant, although a trained eye can easily separate synthetic moissanite from diamond.
Contents
Diagnostics
Synthetic moissanite can be easily separated from diamond by the doubling of facets, due to its high birefringence. Synthetic moissanite is usually cut with the table cut perpendicular to the optic axis, so care should be taken to observe the stone from an angle to the table.
Other clues are parallel needles (along the optic axis) and pinpoint inclusions in synth. moissanite.
Refractometer
Synthetic moissanite has a refractive index higher than diamond and the standard gemological refractometer will not be of any help.
The "dot test" will not help much either, but it will separate synthetic moissanite from zircon.
Dispersion
The dispersive power of synthetic moissanite is about 2.5 times higher than that of diamond and the excessive "fire" should raise a red flag immediately.
Specific gravity
The specific gravity of moissanite is lower than that of diamond and it will float in methylene iodide (SG = 3.33) while diamond will sink in it.
Magnification
The doubling of facets in facetted stones will easily separate synthetic moissanite from diamond.
Typical inclusions include:
- Parallel needles along the optic axis
- Pinpoint inclusions, often in clusters
Polariscope
Synthetic moissanite is uniaxial so it will show an anisotropic reaction under crossed polars.
The interference pattern is uniaxial with a positive optic sign (like zircon).
Spectrum
There is a usual absorption in the violet from 400 to 425nm and this could be confused with the 415nm line in cape series diamond.
Electronic testers
Most electronic diamond testers like from Presidium are created to test the thermal conductivity of the gem. Synthetic moissanite is a good thermal conductor like diamond and this test will fail with the usual testers.
There is a special moissanite tester on the market that does distinguish between diamond and synthetic moissanite, but there are far less expensive techniques available to make the distinction.
References
- Synthetic moissanite: A new man-made jewel - Kurt Nassau, Current Science Vol 79 No 11, 10th Dec 2000 (Indian Academy of Science).
- Scottish Gem Lab News June 1998 by Alan Hodgkinson
Next: Synthetic Cubic Zirconia
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