cassiterite, heavy, metallic, hard tin dioxide (SnO2) that is the major ore of tin. It is colourless when pure, but brown or black when iron impurities are present. Commercially important quantities occur in placer deposits, but cassiterite also occurs in granite and pegmatites. Early in the 15th century, the cassiterite veins in Saxony and Bohemia were mined for tin; peak production occurred there in the 17th century. In the 18th and much of the 19th centuries, the very large vein deposits of Cornwall were the major source of tin. Today most of the world’s cassiterite is mined in Malaysia, Indonesia, Bolivia, Nigeria, Myanmar (Burma), Thailand, and parts of China; other countries produce smaller amounts. For detailed physical properties, see oxide mineral (table).