Round Black Star Sapphire Cabochons

Star Sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism. Star sapphires contain intersecting needle-like inclusions that cause the appearance of a six-rayed'star shaped pattern.

The inclusions are typically the mineral rutile, but in black star sapphire it is inclusions of hematite that cause the star effect.  To cause a star, the inclusions form in parallel and tightly packed groups. The reflection from each group of crystals forms a straight line which appears to float on the surface of the stone.

There are always three groups of crystals which intersect at 60 degree angles from each other. The three lines connect in the center to form the appearance of a 6 ray star.