Introduction Colour range of Australian sapphires . Each of these 41 brilliant cut gems is about 3 mm in diameter. R30650. Source: Geoscience Australia Sapphires and rubies are both gem varieties of the mineral corundum. They have the same chemical composition and structure. Gems generally get their colour because of certain metals or impurities contained in the mineral. The impurities in corundum gems produce the large range of colours found. Sapphire and ruby crystals can grow to large sizes (crystals have been found weighing more than a kilogram). Sometimes they contain inclusions of crystal needles producing a six or twelve pointed star reflection. These are called star sapphires or rubies. Sapphires Sapphires contain traces of iron, titanium and nickel, giving them a range of colours including green, yellow, blue, orange, black and pink. Blue sapphires are the most common and gems with a rich blue to violet colour are the most desirable. Used alone, the word 'sapphire'