A Ruby is a red variety of the gemstone corundum, the color caused mainly by chromium. Rubies are mined in Africa, Asia and Australia. They are most often found in Myanmar, Sri Lanka and Thailand, though they have also been found in Montana and South Carolina; rubies are exceptionally rare, though artificial ones can be manufactured. Sometimes spinels are found along with rubies in the same geological formations and are mistaken for the more valuable gem.
Ruby gemstones are valued according to several characteristics including size, colour, 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)--then it is suspected of being artificially made and is therefore no longer a priceless gem. Most manufactured rubies have dopants added to them so that they can be identified as artificial.
See also: List of minerals