Synthetic moissanite

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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.

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



Next: Synthetic Cubic Zirconia

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