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Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium.〔〔 It is a rock-forming mineral. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen. The name "corundum" is derived from the Tamil word ''Kuruvindam'' or Sanskrit word ''Kuruvinda'' meaning ''ruby''. Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large machines used in machining metals, plastics, and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with an average Mohs hardness of 8.0. In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, which is very high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.〔(The Mineral Corundum ). galleries.com〕 ==Geology and occurrence== Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites.〔 It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering.〔 The largest documented single crystal of corundum measured about , and weighed . The record has since been surpassed by certain synthetic boules.〔(Rubicon Technology Grows 200kg “Super Boule” ), ''LED Inside'', April 21, 2009〕 Corundum for abrasives is mined in Zimbabwe, Russia, Sri Lanka and India. Historically it was mined from deposits associated with dunites in North Carolina, USA and from a nepheline syenite in Craigmont, Ontario.〔 Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York, USA. Abrasive corundum is synthetically manufactured from bauxite.〔 Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China. The surfaces of the axes are remarkably smoothly polished.〔 〕 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「Corundum」の詳細全文を読む スポンサード リンク
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