Moissanite is silicon carbide, one silicon atom for every carbon atom, in a hexagonal crystal. It is a diamond simulant, and every moissanite in jewelry is laboratory-grown.
In more detail
Its refractive index is 2.648 to 2.691, its dispersion 0.104, its specific gravity 3.22 and its hardness Mohs 9.25. It is doubly refractive (birefringence 0.043), so facet edges seen through it can double. Natural moissanite occurs only as tiny crystals; GIA reported the largest known as 4.1 mm. A thermal tester reads moissanite as diamond.
On a grading report
GIA issues only an Identification Report for moissanite. Since May 2024, IGI has graded one producer's moissanite. Other moissanite gets an identification report, a certificate of authenticity or a seller's card.
Related terms
- Simulant: A simulant is a material that looks like a gem but has a different chemistry, structure and properties. Cubic zirconia, moissanite and glass are diamond simulants.
- Facet doubling: Facet doubling is the double image of facet edges seen through a strongly doubly refractive stone. It shows clearly in moissanite under a 10× loupe.
- Thermal diamond tester: A thermal diamond tester measures how fast a stone draws heat from a hot tip. Diamond conducts heat very well, so it reads “diamond”.
- Specific gravity: Specific gravity compares a stone's weight with the weight of the same volume of water. It decides how much a stone of a given size weighs.
- Dispersion: Dispersion is how differently a stone bends red and violet light. Gemologists give it as the index difference between the B line (686.7 nm) and the G line (430.8 nm).
- Birefringence: Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.