The Collaborative International Dictionary of English v.0.48:

Rutile \Ru"tile\ (r[udd]"t[i^]l), n. [L. rutilus red, inclining
   to golden yellow.] (Min.)
   A mineral usually of a reddish brown color, and brilliant
   metallic adamantine luster, occurring in tetragonal crystals.
   In composition it is titanium dioxide, like octahedrite and
   brookite.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rutile
    n 1: a mineral consisting of titanium dioxide in crystalline
         form; occurs in metamorphic and plutonic rocks and is a
         major source of titanium