Asterism (gemology)
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"Star ruby" redirects here. For the grapefruit variety, see Grapefruit § Star Ruby.
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).
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/e/ee/Star-Saphire.jpg/200px-Star-Saphire.jpg)
![A 6.33 carat natural purple-pink star sapphire is mounted in a platinum setting.](http://upload.wikimedia.org/wikipedia/commons/thumb/3/30/-_star_sapphire.png/640px--_star_sapphire.png)
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.