Laboratory-Grown-Moissanite

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image 2 - Laboratory-Grown-Moissanite

In the gem and jewellery trade, “moissanite” is a laboratory-grown gemstone that is commonly used as a simulant for diamond. However, in the last few years, it has become an independent gem in the market because of its durability, high dispersion, colour (or lack thereof) and availability. Natural moissanite does exist but only as tiny grains not suitable for jewellery.
Moissanite is made up of tetrahedrally coordinated silicon carbide (SiC). It occurs in many different polytypes, and each polytype has its own properties. Polytypism is a 1-dimensional type of polymorphism. Polytypes of moissanite all have the chemical formulae – “SiC”, but their atomic stacking structure differs. The most commonly encountered polytypes of moissanite in the gem trade are the 4H and 6H polytypes. The number (4 or 6) refers to the number of layers in a stacking sequence, ‘H’ stands for ‘Hexagonal’ and refers to the crystal lattice type.

(In this article, when referring to “moissanite”, we are referring to the 4H and 6H polytypes because those are grown in a laboratory for use as near-colourless gems.)
The preferred growth method for moissanite is the seeded sublimation process (growing solid crystals from vapour – also known as the Physical Vapour Transport (PVT) method). The specific method used for bulk single crystal 4H and 6H polytypes is usually the Lely method or a modified version. This is done in a graphite crucible at low pressures (around 10-3 Pa) and temperatures up to 2600°C. SiC powder is used as a source material, and depending on specific growth environments, the growth rate can be as high as 2 mm/hour. By changing the Si:C ratio and pre-treating the source powder, the polytype outcome can be controlled.
Different dopants are used to alter the properties of the stone – such as colour – and these are introduced at various stages of the growth process.

The most common dopants used are usually N, P, As, Al, B and/or Ga.
Moissanite was originally grown by Cree Research Inc. in the USA for use in the gem trade in 1996, but after their patents expired, Russia and China became major producers. Research has led to faster growth rates, lower defect densities and much less colour, and a preference has been given to the 4H polytype.
Moissanite was originally introduced into the market as a cheaper alternative to natural diamonds but has become a very popular gem in its own right. Moissanite does have a very similar appearance to diamond, and it is almost impossible to separate the two without the correct equipment. In the past, electrical conductivity meters have been used to separate moissanite from diamonds, but the latest ‘low-conductivity’ moissanites behave similarly to diamond, and therefore these testers should not be solely relied upon.

How Can I Identify Whether The Stone in Front of Me is a Moissanite? Here Are Some Tips:

  1. Check the (long wave) fluorescence: Almost all commercial moissanites either have no fluorescence or very faint orange fluorescence.
  2. If the stone is loose, you can use a heavy liquid to determine the relative specific gravity (SG) of the stone. Stabilised diiodomethane has an SG of 3.32; moissanite has an SG of 3.21, and diamond has an SG of 3.52. Therefore, a diamond will sink in the liquid, and moissanite will float in the liquid.
  3. Probably the most reliable way to identify moissanite is using magnification (preferably a microscope, but a loupe can work too) to observe the inclusions, etc. Moissanite exhibits strong double refraction and often has micropipes (stringers) and bubbles/negative crystals as inclusions. The girdle of moissanite is generally striated diagonally and has a melted appearance.

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All images courtesy of Taryn Khourie

Should Moissanite be Graded and Which Terminology Should be Used?

Absolutely! It is important to grade all gemstones so that the quality can be assessed and sorted/priced accordingly. Very important: Anything other than diamond – including moissanite – should not be graded using diamond terminology (D-Z for colour; IF, VVS, etc. for clarity); instead, terms such as “Eye Clean” and “Near Colourless” should be used. To the right are tables demonstrating the correct terminology that should be used
in relation to ‘diamond terminology’:

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If in doubt, always send your stone to a capable gemological laboratory for testing and grading.

All rights reserved. This article is the sole property of Kaylan Khourie and may not be reproduced by any form or means without the express written consent of Kaylan Khourie. Unless otherwise stated, all images have been captured by and belong to the author.