Ruby is a red variety of corundum, an aluminium oxide with a hardness of 9. Its crystal system is hexagonal (trigonal). It forms in prismatic, tabular, bipyramidal or rhombohedral crystals. It can also occur in granular or massive habits. Until 1800, when Ruby was recognized as a variety of corundum, red Spinels and Garnets were also believed to be Ruby. The intense red colour of Ruby comes from chromium. In some Rubies, inclusions of titanium oxide cause light diffusion in cabochon-cut stones that can appear as a six-pointed star. Rubies have been mined in Sri Lanka, Burma, Thailand, India, Brazil and the United States.