Tiger's eye  is a chatoyant gemstone that is usually yellow- to red-brown, with a silky luster. It is a fibrous silicified crocidolite (blue asbestos), a classic example of pseudomorphous replacement. An incompletely silicified blue variant is called Hawk's eye. A member of the quartz group, its physical and optical properties are identical or very near to those of single-crystal quartz. The gems are usually cut en cabochon in order to best display their chatoyancy. Red stones are brought about through gentle heat treatment. Honey-coloured stones have been used to imitate the much higher valued cat's eye chrysoberyl, but the overall effect is unconvincing. Artificial fibreoptic glass is a common imitation of tiger's eye, and is produced in a wide range of colours. Tiger iron is an altered rock composed chiefly of tiger's eye, red jasper and black hematite. The undulating, contrasting bands of colour and luster make for an attractive motif, and it is mainly used for jewelry-making and ornamentation. Tiger iron is a popular ornamental material used in a variety of applications, from beads and cabochons to knife hilts.