Rutile

The commonest natural form of titanium dioxide (TiO2), rutile is an important ore of titanium that crystallizes in the tetragonal system. It appears in prismatic, acicular, or geniculated twinned crystals with an adamantine luster, a light-brown streak, good {110} cleavage, and colors from reddish-brown and red to black and golden. Its Mohs hardness is 6 to 6.5 and its specific gravity 4.18 to 4.25. Golden needle-like rutile crystals enclosed in quartz, known as rutilated quartz, are cut as gemstones, and fine rutile inclusions produce the silk that gives rise to star rubies and sapphires. A high specific gravity and adamantine luster distinguish rutile from the darker oxides it can resemble.

Rutileは二酸化チタンの最も一般的な天然形態であり、チタンの重要な鉱石である。石英中に包含された金色の針状結晶（ルチルクォーツ）は宝石として珍重される。スタールビーやスターサファイアの「シルク」包有物としても産出する。