Red variety of the mineral Corundum, which is composed of aluminium oxide. The red colour is caused mainly by chromium and titanium. It is natural for rubies to have imperfections, including colour impurities. The origin of the name comes from the Latin word "ruber" for red. Natural occurring rubies are very rare and extremely rare in large sizes over 3 carats. They can be found in many regions around the world such as India, East Africa, South America, and a few deposits in the United States. Source: Encyclopedia of gemstones and minerals.