Moissanite

A crystalline form of silicon carbide, moissanite can also occur naturally. However, the natural material is extremely rare and too small to facet. The moissanites used as diamond simulants are produced in labs. Strictly speaking, they're synthetic moissanites, but you'll frequently find them referred to simply as "moissanite." Synthetic moissanite engagement ring. Photo by Angela. Public Domain.

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Credit: Angela