![]() Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. It also occurs in low-silica igneous syenite and nepheline syenite intrusives. Geology and occurrence Ĭorundum from Brazil, size about 2 cm × 3 cm (0.8 in × 1 in)Ĭorundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. In addition to its hardness, corundum has a density of 4.02 g/cm 3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low- atomic mass elements aluminium and oxygen. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. īecause of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. The name "corundum" is derived from the Tamil- Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda). A rare type of sapphire, padparadscha sapphire, is pink-orange. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. Corundum has two primary gem varieties: ruby and sapphire. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. ![]() May fluoresce or phosphoresce under UV lightīlack granular corundum intimately mixed with magnetite, hematite, or hercyniteĬorundum is a crystalline form of aluminium oxide ( Al 2O 3) typically containing traces of iron, titanium, vanadium and chromium. ![]() May alter to mica on surfaces causing a decrease in hardness Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular CorundumĬolorless, gray, golden-brown, brown purple, pink to red, orange, yellow, green, blue, violet may be color zoned, asteriated mainly grey and brown
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