Nobel prize winner Henri Moissan discovered natural silicon carbide inside a Canyon Diablo meteorite. The substance was then named moissanite. After much research and development, synthetic moissanite (synthetic silicon carbide) was able to be grown into large crystals for jewelry applications. Before that, silicon carbide was used for decades as an industrial abrasive (and still is).
Using the Relative Mohs Hardness Scale, it is rated 9 1/4. This would make it harder than corundum, but much softer than diamond. Remember the hardness range between 9 and 10 far exceeds the difference in hardness between 1 and 9 on the scale. Hence moissanite is still a very hard substance, producing much sharper facet edges than for softer stones (and will remain so).
Diamond's specific gravity is 3.52, whereas moissanite's is 3.22. This indicates that moissanite is 91% of diamond's weight, given the exact same proportions (or volume). So an easy way to identify or confirm the authenticity of moissanite would be to use the diamond weight estimation formula (discussed in greater detail in section 11).
For example, suppose the stone in question measures:
6.41 - 6.43 x 3.80 mm
Average diameter: 6.42 Depth: 3.80
Formula would read:
6.42 x 6.42 x 3.80 x .0061=0.96 carat for a diamond.
Your carat scale shows a weight of 0.87 carat which would indicate a moissanite since it is 91% of the estimated diamond weight. At the very least you would be sure it is not diamond.
Clarity in moissanite varies from many needle-like inclusions arranged in directions parallel to the pavilion surfaces, to high clarity (clean).
The color of moissanite also varies, but it will usually be obviously tinted in the subtle yellows, browns, greens, greys or mixtures thereof. Undoubtedly C3, the manufacturer of moissanite, will try to further improve this color for obvious reasons.
Polishing lines generally all run in the same direction on moissanite, since grain direction does not have to be taken into account in the polishing process. Remember that a diamond's polishing lines will always run in multiple directions.
Rounded facet edges are another indicator of moissanite, but with better manufacturing methods and more skillful cutters, this will not be sufficient to confirm your identification.
Natural diamond-like inclusions never appear in moissanite, but of course, if the stone in question has a very high clarity, this alone would not be definitive.
Moissanite is doubly refractive. This double refraction -- which must be carefully watched for -- can be observed through a bezel facet, directing your line of sight toward the culet. The culet will appear slightly doubled. Be aware that polished moissanites are oriented in a fashion, that makes the table view (line of sight through the center of the table, to the culet) appear singly refractive.
The older generation of diamond testers cannot be used to test moissanite. C3 (and other gem instrument vendors) sells a tester specifically for moissanite. It is an electronic device similar to diamond testers. The author of this course does not recommend the use of testers, since they are not 100% reliable. As with any other electronic device, a moissanite or diamond tester can become defective.
With all the testing methods and your knowledge of diamond identification, moissanite should not be a problem, as long as you are thorough in your identification procedures.