Corundum is a crystalline form of Aluminium oxide with traces of Iron, Titanium and Chromium. It is a rock forming mineral which is naturally transparent but can have different colours when impurities are present. Used as a gemstone, red corundum is known as Ruby whilst all other colours are known as Sapphire.
It is the most common naturally occurring crystalline form of Aluminium oxide, much less common are gem quality Rubies and Sapphires. The deep red colour of Ruby is caused by traces of the metallic element Chromium and the other impurities such as Iron and Titanium cause the varying colours of the Sapphire.
Due to its hardness, (in its pure form, Corundum measures 9 on Mohs scale of mineral hardness) it can scratch almost every other mineral. Corundum is commonly used as an abrasive on everything from sandpaper to large machines used for machining metals, plastics and wood. Some emery (the material used on nail files) is a mix of Corundum and other substances which make it less abrasive.
Corundum dates back to approximately 500 million to 2 billion years ago when it was first formed in the Earth’s upper mantle some 60–400 km deep inside the Earth under conditions of immense temperatures and pressures. With the rising magma it was brought to the Earth's surface where the cooling of the magma produced igneous rocks with the Corundum incorporated within it. The action of weathering agents such as temperature, rainfall etc over millions of years, caused the separation of the Corundum from the parent rocks which were later washed down the hills and slopes and deposited in the low lying areas as alluvial deposits (not cemented together into a solid rock) and this is where it is mined today in various parts of the world including Afghanistan and Sri Lanka.
Afghanistan has been famous for its Rubies and Spinels since ancient times as has Sri Lanka with some being traced back to as early as the period of King Solomon in the 10th century B.C.
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