The Sanskrit name for diamond is vajra, meaning impenetrable, also thunderbolt which is indicative of instant illumination. The Persian almasfor diamond means as invincible. The word diamond has, however, been derived from the Latin adamas, from Greek for unconquerable. Diamond is the crystalline form of carbon. It is found in nature as water-worn pebbles, or grains in river-gravels and other alluvial deposits. The diamond is one of the most popular gemstones, owing chiefly to its lustre. It is the hardest substance known and, therefore, is used as an abrasive.