Corundum: History, Varieties, Properties, and Uses

Corundum is a mineral that has been known since ancient times for its beauty and hardness. Here is some information about corundum:

• History: Corundum was first described in the writings of classical antiquity, where it was called "sapphire" and was believed to have healing properties. The ancient Greeks and Romans used corundum as a gemstone and for decorative purposes. In the 19th century, large deposits of corundum were discovered in various parts of the world, leading to increased industrial use of the mineral.
• Varieties:
• Ruby: red corundum
• Sapphire: any corundum that is not red
• Padparadscha: pink-orange sapphire
• Color-change sapphire: sapphire that appears blue in natural light and purple in incandescent light
• Star corundum: corundum that exhibits asterism (a star-like pattern) when cut en cabochon
• Origin: Corundum can be found in many parts of the world, including India, Russia, Australia, and the United States. It is formed in metamorphic rocks such as mica schist, gneiss, and marble.
• Physical Properties:
• Hardness: 9 on the Mohs scale, making it one of the hardest minerals
• Density: 3.95-4.1 g/cm³
• Color: can be red, blue, green, yellow, brown, black, or colorless
• Luster: vitreous to adamantine (diamond-like)
• Transparency: transparent to opaque
• Crystal system: trigonal
• Cleavage: none
• Fracture: conchoidal (like glass)
• Optical Properties:
• Refractive index: 1.762-1.778
• Birefringence: 0.008-0.010
• Elementary Analysis:
• Chemical formula: Al₂O₃
• Molecular weight: 101.96 g/mol
• Composition: Aluminum (52.93%), Oxygen (47.07%)
• Most Expensive Piece Selling Record: The 55.22-carat Estrela de Fury from Mozambique, the largest ruby to have been sold at a public auction, achieved a hammer price of US$30 million after 90 seconds of bidding
• The most expensive corundum ever sold is the Sunrise Ruby, a 25.59-carat Burmese ruby that sold for $30.42 million at a Christie's auction in 2015.
• Uses:
• Gemstones: Corundum's various colors, including ruby and sapphire, are highly prized as gemstones.
• Abrasive: Corundum's hardness makes it an excellent abrasive for grinding and polishing. It is used in many industrial applications, including sandpaper and grinding wheels.
• Refractory material: Corundum's high melting point and resistance to chemicals make it useful as a refractory material in high-temperature applications. It is used in the production of ceramics, glass, and steel.
• Electronics: Corundum is used as a substrate for electronic devices such as LEDs and lasers.
• Fashion: Corundum jewelry is a popular choice for its durability and beauty. It is often used in engagement rings and other high-end jewelry.