Moissanite

Moissanite is a silicon carbide that is sometimes seen as a diamond simulant. In nature, it is found only as tiny crystals, primarily in meteorites.

Early diamond testers checked diamond's thermal conductivity, which is very high. But moissanite has a thermal conductivity similar to diamond, which fooled quite a number of people who had come to rely on the testers. Modern diamond testers also check electrical conductivity, which can separate diamond from moissanite.

Visually, the two are very easy to distinguish. Moissanite is doubly refractive, unlike diamond. When looking through the stones from the right angle it is very easy to spot double images of facet junctions through the stone. This is a dead giveaway that you don't have a diamond in your hand.