an optical phenomenon whereby star-like rays of reflected light are observed on the surface of some gems or minerals, when cut cabochon and viewed in reflected or transmitted light from a certain direction. Caused by the presence of minute, almost microscopic inclusions, arranged in a regular (generally in three identical crystallographic directions as hexagonal) series, parallel to the prism face, in some varieties of gemstone it is referred to as silk. A cabochon cut from such a stone exhibits a ray of light on the surface of mineral, perpendicular to each parallel set of inclusions. This three-direction cut with a chatoyancy effect is known as asterism, for example in rubies and sapphires. In rubies and sapphires, the microscopic rutile needle inclusions responsible for these effects are arranged in two or more intersecting bands of light that radiate from a bright center. In the case of the 6-rayed star, it is the presence of fine parallel fibers or crystal inclusions or channels...
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© 2009 Springer-Verlag Berlin Heidelberg New York
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(2009). asterism. In: Manutchehr-Danai, M. (eds) Dictionary of Gems and Gemology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-72816-0_1374
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