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Ruby is a rare gemstone variety of Corundum, the other famous variety being Sapphire. Corundum by itself is colorless, being made of only aluminum and oxygen, but Ruby is distinguished by its rich red color. This comes from small amounts of chromium replacing some of the aluminum.
Natural Ruby often has tiny needle-like crystals of Rutile and other inclusions, which helps distinguish them from synthetics and imitations. Gem-variety Ruby that is translucent enough to cut gemstones out of is exceedingly rare, and Ruby is still considered one of the four traditional precious stones. In addition to its rarity, it earned this distinction by its hardness, with only Diamond and Moissanite being harder.
