Ruby is a precious variety of Corundum, a high hardness mineral, losing only to Diamond. Ruby red color is caused by lower Cr (chrome) quantities, replacing Al (aluminum) in crystal structure.
This corundum variety is used as gemstone, but it can also be blue, and this other variety is known as Sapphire.
Due to its high hardness, it was widely used as abrasive and for polishing, and it has been modernly replaced by synthetic abrasives. It is also used in precision mechanisms, including special watches and engines. Mineral, gemological and collection species
Source: MM Gerdau - Museu das Minas e do Metal -