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Asterism (gemmology)

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Asterism on the surface of a blue star sapphire. This is the 182-carat (36.4 g) Star of Bombay, housed in the National Museum of Natural History, Washington D.C.

In gemmology, an asterism is an optical phenomenon displayed by some rubies, sapphires, and other gems (i.e. star garnet, star diopside, star spinel, etc.) of an enhanced reflective area in the shape of a "star" on the surface of a cabochon cut from the stone. Star sapphires and rubies get their asterism from the titanium dioxide impurities (rutile) present in them.

A distinction can be made between two types of asterism:

• Epiasterism, such as that seen in sapphire and most other gems, is the result of a reflection of light on parallel arranged inclusions inside the gemstone.
• Diasterism, such as that seen in rose quartz, is the result of light transmitted through the stone. In order to see this effect, the stone must be illuminated from behind.

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