Topaz is a common gemstone that has been used for centuries as jewelry. The stone is one of the hardest minerals, and is the hardest silicate mineral found in nature.  A silicate of aluminum, topaz contains about 20 percent water and fluorine. The relative proportions of these impurities are responsible for the color of the stones.  Crystals with more water are yellow to brown, while those with more fluorine are typically blue or colorless, these crystals are colorless to brown in hue.  Size is 3 to 11 carats each.