Asterism (gemology)

Asterism (from Ancient Greek: ἀστήρ star), or star stone, is a name applied to the phenomenon of gemstones exhibiting a luminous star-like shape when cut en cabochon. The typical asteria is the star sapphire, generally a bluish-grey corundum, milky or opalescent, with a star of six rays. In red corundum the stellate reflection is less common, and hence the star-ruby occasionally found with the star-sapphire in Sri Lanka is among the most valued of "fancy stones". When the radiation is shown by yellow corundum, the stone is called star-topaz. Cymophane, the chatoyant chrysoberyl known as cat's eye, may also be asteriated. In all these cases the asterism is due to the reflection of light from twin-lamellae or from fine tubular cavities or thin enclosures definitely arranged in the stone. The astrion of Pliny the Elder is believed to have been a moonstone, since it is described as a colourless stone from India having within it the appearance of a star shining with the light of the moon. Star-stones were formerly regarded with much superstition.