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Corundum
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<p>Corundum is a <a href="/facts/Crystal/e2R2mk3E">crystalline</a> form of <a href="/facts/Aluminium_oxide/wXM9P6pI">aluminium oxide</a> (Al2O3) typically containing traces of <a href="/facts/Iron/DkICCKIC">iron</a>, <a href="/facts/Titanium/53ArnDoy">titanium</a>, <a href="/facts/Vanadium/UvKkKTSm">vanadium</a>, and <a href="/facts/Chromium/6sVk6Uu7">chromium</a>. It is a <a href="/facts/Rock_(geology)/FXCJkmop">rock</a>-forming <a href="/facts/Mineral/LkUFbNmv">mineral</a>. It is a naturally <a href="/facts/Transparency_and_translucency/PRryNNW6">transparent</a> material, but can have different colors depending on the presence of <a href="/facts/Transition_metal/0slGWoLQ">transition metal</a> impurities in its crystalline structure. Corundum has two primary <a href="/facts/Gemstone/kHffmvE3">gem</a> varieties: <a href="/facts/Ruby/faGnqzW8">ruby</a> and <a href="/facts/Sapphire/C11GJAmm">sapphire</a>. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, <a href="/facts/Sapphire/C11GJAmm">padparadscha</a> sapphire, is pink-orange. </p><p>The name "corundum" is derived from the <a href="/facts/Tamil_language/7I0lHL0d">Tamil</a>-<a href="/facts/Dravidian_languages/s8pMU64e">Dravidian</a> word <i>kurundam</i> (ruby-sapphire) (appearing in <a href="/facts/Sanskrit/ychfjhlx">Sanskrit</a> as <i>kuruvinda</i>). </p><p>Because of corundum's hardness (pure corundum is defined to have 9.0 on the <a href="/facts/Mohs_scale/wXBMcVIm">Mohs scale</a>), it can scratch almost all other minerals. <a href="/facts/Emery_(rock)/vjqXkfaI">Emery</a>, a variety of corundum with no value as a gemstone, is commonly used as an <a href="/facts/Abrasive/UJ9TvVMx">abrasive</a> on <a href="/facts/Sandpaper/tWd80gSw">sandpaper</a> and on large tools used in machining metals, plastics, and wood. It is a black granular form of corundum, in which the mineral is intimately mixed with <a href="/facts/Magnetite/SVxoGUs2">magnetite</a>, <a href="/facts/Hematite/716YBzGr">hematite</a>, or <a href="/facts/Hercynite/stneoKDS">hercynite</a>. </p><p>In addition to its hardness, corundum has a density of 4.02 g/cm3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-<a href="/facts/Atomic_mass/J1FJY9MV">atomic mass</a> elements <a href="/facts/Aluminium/7wmR8jYE">aluminium</a> and <a href="/facts/Oxygen/acyn6o8r">oxygen</a>. </p>