Corundum is composed of aluminum and oxygen. It is a very simple compound but has many different appearances. Pure corundum is colorless. More often than not other elements replace the aluminum or get trapped in the crystal lattice and cause different colors. When chrome replaces some of the aluminum it turns the crystal red. The more replacement the redder it gets to a certain extent, after that it becomes a different mineral. Deep red corundum is called a ruby. The richer the red and the clearer the crystal the more valuable it becomes. The name ruby comes from the Latin word "ruber" in reference to the red color. In biblical times it was called carbuncle. Which was a word used to describe any red gemstone since they didn't have the analytical skills to differentiate between minerals. Ruby and red spinel were often confused and many of the ancient stones that were called ruby or carbuncle were actually spinel as it was a more common mineral.
Iron & titanium can also replace aluminum in varying amounts in the structure, causing the crystals to turn shades of blue, yellows, pinks, golds, and a myriad of other colors. These are known as sapphires. Orange-pink (no trace of brown) sapphires are called padpadradscha, named after the lotus flower in Sanskrit.
Today it is a safe bet to assume all rubies and sapphires have been treated with at least one method to enhance the color and/or clarity.
Where corundum forms on earth is dictated by the geology. Corundum forms in iron and manganese rich siliceous environments that are low in silica and rich in alumina. This is why we don't find corundum in the west U.S.. We have mostly silica sand and granites rich in silica.
The name corundum is a Sanskrit word meaning "hard stone". Centuries ago they realized it was harder than anything but a diamond.
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