ARNOLD FISHER / SCIENCE PHOTO LIBRARY ARNOLD FISHER / SCIENCE PHOTO LIBRARY
Sample of sapphire. Sapphire is a form of corundum (aluminium oxide), with a colourless or blue colour. Corundum is an accessory mineral of igneous rocks which are under-saturated with respect to silica, or in high-grade metamorphic rocks poor in silica and rich in aluminium (eg marble). Clear, coloured varieties of corundum are highly-prized as gemstones. Red coloured corundum is known as ruby, green coloured corundum is known as emerald. This sample was found in Sri Lanka.
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