Quartz, silicon oxide ('silica'), is one of the most abundant and ubiquitous minerals in the earth's crust. It is the primary constituent of beach sand and occurs in many colors, all of which are used as gemstones. Silica also may percolate through rocks and replace other minerals on a molecular basis. When this occurs with asbestos the resulting material is hard and displays a 'sheen' due to light reflecting from the fibrous structure of the original asbestos (crocidolite). This has been trade named 'tiger eye', or 'tiger's-eye', and is generally brown in color. Blue material is rare and is sometimes called 'Hawks-eye'. Red tiger eye is produced by heating the brown material. Tiger eye is generally cut as cabochons (dome-shaped stones) to maximize the reflective sheen.