Ruby is a red gemstone, a variety of the mineral corundum (aluminium oxide) in which the color is caused mainly by chromium. Its name originates from ruber, Latin for red. Natural rubies are exceptionally rare, though artificial ones (sometimes called created ruby) can be manufactured which are comparatively inexpensive.

Rubies are mined in Africa, Asia, Australia and Greenland. They are most often found in Myanmar, Sri Lanka and Thailand, though they have also been found in Montana and South Carolina. Sometimes spinels are found along with rubies in the same geological formations and are mistaken for the more valuable gem. However, fine red spinels may approach the average ruby in value.

Rubies have a hardness of 9 on the Mohs scale of mineral hardness, and among the natural gems are only surpassed by diamonds in hardness. Other varieties of corundum are called sapphires.

Ruby gemstones are valued according to several characteristics including size, color, clarity and cut. All natural rubies have imperfections in them. On the other hand, artificial rubies may have no imperfections. The fewer the number and the less obvious the imperfections, the more valuable the ruby is—unless there are no imperfections (i.e., a "perfect" ruby), in which case it is suspected of being artificially made and its status as a priceless gem is therefore not assured. Some manufactured rubies have dopants added to them so that they can be identified as artificial, but most require gemological testing to determine their origin.

A synthetic ruby crystal was used to create the first laser.

Ruby is the birthstone associated with July.

The world's biggest star ruby is the Rajaratna Ruby, which weighs 2,475 carats (495 g). The world's biggest double-star ruby (with a 12-pointed star) is the Neelanjali Ruby, weighing 1,370 carats (274 g). Both rubies currently belong to G. Vidyaraj from Bangalore in India.

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