Rutile

Rutile is a mineral composed of titanium dioxide which occurs in three distinct forms, as:

• rutile, a tetragonal mineral usually of prismatic habit, often twinned;
• anatase or octahedrite, a tetragonal mineral of octahedral habit; and
• brookite, an orthorhombic mineral. Both octahedrite and brookite are relatively rare minerals.

Rutile has a sub-conchoidal fracture, is brittle, has a metallic-adamantine lustre, and is commonly reddish brown but also sometimes yellowish, bluish, or violet. It is transparent to opaque. Rutile may contain up to 10% iron. Rutile is the most stable form of titanium dioxide and is produced at the highest temperatures, with brookite being formed at lower temperatures and octahedrite being formed at still lower temperatures.

Rutile is found as an accessory mineral in many kinds of igneous rocks, and to some extent in gneisses and schists. In groups of acicular crystals it is frequently seen penetrating quartz as in the "fléches d'amour" from Grisons, Switzerland. Small rutile needles present in gems are responsible for "star" sapphires, "star" rubies and other "star" gems, an optical phenomenon known as asterism.

Synthetic rutile was first produced in 1948 and is sold under a variety of names. It has very high dispersion and refracts light so strongly that it looks phoney, being very colorful. Synthetic rutile can be made in a variety of colors, but not a pure transparent white, being always slightly yellow. Because of its phoney appearance it is seldom used in jewellery. It is not very hard, only about 6 on the Mohs hardness scale. The near colorless diamond substitute is sold under the name Titania.

Rutile derives its name from the latin rutilus, red, in reference to the deep red color observed in some specimens when viewed by transmitted light. Natural rutile is usually opaque or very dark red.

See also: List of minerals