Aluminum

Aluminum is the third most abundant element in the earth's crust, after only oxygen and silicon, and it is the most abundant of all metals. It constitutes 8.1% of the crust by weight and 6.3% of all the atoms in the crust. Because it is a very active metal, aluminum is never found in the metallic form, but only in a wide variety of earthy and rocky minerals, including feldspar, mica, granite, and clay. Kaolin is an especially fine, white aluminum-containing clay that is used in making porcelain.

Aluminum oxide, Al₂O₃, often called alumina, does not melt until over 3,632°F (2,000°C), and is used to line furnaces. Other forms of alumina are corundum and emery, which are very hard and are used as abrasives. Among the many other mineral forms that aluminum is found in are several semiprecious gemstones, including garnet (Fe₃Al₂Si₃O₁₂), beryl (Be₃Al₂Si₆O₁₈), and ruby and sapphire, which are Al₂O₃ containing impurities of chromium and iron, respectively. Artificially made rubies and sapphires are used in lasers.