![]() One carat = 200 milligrams (1/5 of a gram).Ī solid mineral enclosed by symmetrically arranged planes. Birefringent gemstones have two different refractive indices this makes the optical phenomenon very useful for gemstone dealers to correctly identify certain man-made fakes from real gemstones. The splitting of a single ray of light into two rays (also referred to as double refraction). Glossary Basaltĭark coloured, fine-grained volcanic igneous rock, low in silica content, composed largely pyroxene, and olivine and feldspar. Intense metamorphism may create garnet, emerald, jade and lapis lazuli. Pegmatite, an intrusive igneous rock, may concentrate rare minerals to form gemstones such as beryl, ruby, sapphire, tourmaline and topaz. Gemstones are often found in igneous rocks. Unusual geological conditions are required to create gemstones, which is why they are so rare. One carat = 200 milligrams (1/5 of a gram).
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