|Transparency||Transparent to opaque|
|Luster||Adamantine to sub-metalic|
|Cleavage||distinct prismatic and pyramidal|
|Hardness||6 to 6.5|
|Optical Character||Uniaxial+ Double Refractive|
|Refractive index||2.616 - 2.903|
|Streak||Pale yellow to brown|
The natural chemical compound titanium oxide (TiO2) has the distinction of occurring in three different crystal modifications, known as rutile (R00-teel), anatase (ANN-ah-tase) and brookite (BROOK-ite). Both rutile and anatase belong to the tetragonal system, but they differ in axial ratio and habit; rutile crystals are commonly prismatic or bipyramidal and often twinned and striated. In the accompanying lettered photograph, "a" is a striated prism, "b" is a knee-shaped twin crystal, and "c" is a rosette-shaped twin crystal. Brookite, on the other hand, crystallizes in the orthorhombic system. Of the three, only rutile finds limited application as a gemstone, primarily as a curio or collector's item. The color of these minerals ranges from dark brownish red to red to black, sometimes yellowish, and varies from transparent to opaque. The name rutile is derived from the Greek word rutilus, meaning "red".
One of the most striking characteristics of rutile is its very high R.I. (2.616 - 2.903), resulting in a tremendous birefringence of .287. Another notable feature is its very strong dispersion. Other properties are as follows: optic character, uniaxial positive; hardness, 6 to 6 1/2; cleavage, distinct prismatic and pyramidal; luster, adamantine to sub-metallic; streak, pale yellow or brown; S.G., 4.20 to 4.30; fracture, subconchoidal to uneven. It exhibits no phenomenon nor characteristic inclusions. It is affected neither by acids nor by heat.
Rutile, which is the most abundant titanium mineral, forms in gneiss, mica-schist, slate, granite, limestone and dolomite, and is commonly associated with quartz, hematite and feldspar. Notable sources include Arendal and Kragero, Norway; the Ural Mountains, Russia; Binnental and St. Gothard district, Switzerland; Nelson County, Virginia; Graves Mountain, Georgia; Magnet Cove, Arkansas; Keystone, South Dakota; and Oaxaca, Mexico.
Transparent, brownish-red crystals make attractive faceted stones. However, since the mineral is more commonly translucent to semitransparent and dark in color, and thus not permitting maximum light transmission, crown and pavilion angles are not a major concern. Because of the presence of distinct cleavage, sawing sometimes proves difficult. Stones should be polished on a tin lap with Linde A powder as the agent.