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Reference.org
Asterism (gemology)
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<p>An asterism is a <a href="/facts/Diffraction_spike/0JR8UxCH">star-shaped</a> concentration of <a href="/facts/Light/jeofpMn4">light</a> <a href="/facts/Reflection_(physics)/BhcjDs79">reflected</a> or <a href="/facts/Refraction/S8kvSKzM">refracted</a> from a <a href="/facts/Gemstone/kHffmvE3">gemstone</a>. It can appear when a suitable stone is cut <i>en <a href="/facts/Cabochon/e5cUkCt2">cabochon</a></i> (i.e. shaped and polished, not <a href="/facts/Faceting/Yte7F7bh">faceted</a>). </p><p>A gemstone that exhibits this effect is called a star stone or asteria. The best known is <a href="/facts/Sapphire/C11GJAmm">star sapphire</a>, but many other minerals can also be asteria, usually due to impurities in the crystal structure. </p>