Asterism (gemology)

"Star ruby" redirects here. For the grapefruit variety, see Grapefruit § Star Ruby.
Asterism on the surface of a blue star sapphire.
A 6.33 carat natural purple-pink star sapphire is mounted in a platinum setting.
A purple-pink star sapphire displaying asterism in a platinum ring.

An asterism (from Ancient Greek ἀστήρ (astḗr) 'star', and -ism) is a star-shaped concentration of light reflected or refracted from a gemstone. It can appear when a suitable stone is cut en cabochon (i.e. shaped and polished, not faceted).

A gemstone that exhibits this effect is called a star stone or asteria. The best known is star sapphire, but many other minerals can also be asteria, usually due to impurities in the crystal structure.