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More info: Precious stones.
An optical phenomenon of a star-like figure that is seen in some crystals by reflected light or transmitted light. An example is the 6-ray (sometimes 4-ray, 8-ray or 12-ray) star-like figure that is observed by reflected light in some gemstones (especially the star ruby and the star sapphire) when cut en cabochon in such a manner that the greatest thickness of the stone lies parallel to the vertical axis of the crystal.
The effect is caused by the reflection of light from a series of microscopic fibrous inclusions or small canals lying within the crystal parallel to the prism faces and arranged in three directions that intersect, usually at angles of 60°.
Such stars are also seen in some other gemstones, e.g. beryl, almandine, star opal, rose quartz.
A stone showing asterism is known as an asteria or a 'star stone'.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson