Reference work entry
Sapphire is a blue and transparent corundum. Currently, the gemstone trade collectively applies this term to corundum of all colours except ruby. Its chemical composition is aluminium sesquioxide (Al 2O 3), and its blue colour comes from the trace elements titanium (Ti 4+) and iron (Fe 2+). Its crystal structure is hexagonal scalenohedral or cylindrical, and its Mohs hardness is 9, which is second only to that of diamond. Transparent or translucent sapphires with a bright blue colour rank the best, and hibiscus blue or cornflower blue sapphires are considered top-grade sapphires. The largest cut sapphire, called the Logan Sapphire, weighs 423 carats and is now displayed in the Smithsonian Museum of Natural History in Washington D.C., United States. Sapphires are mainly found in alkaline basalt in the form of xenoliths. They are also found in contact metamorphism zones and become placer deposits after weathering. The main mining areas include India, Myanmar, Sri Lanka, Thailand and...
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