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Asterism (from ''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. == Description == 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. The Star-effect or "asterism" is caused by the dense inclusions of tiny fibers of rutile (also known as "silk"). The stars are caused by the light reflecting from needle-like inclusions of rutile aligned perpendicular to the rays of the star. However, since rutile is always present in star gemstones, they are almost never completely transparent. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Asterism (gemology)」の詳細全文を読む スポンサード リンク
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