The Collaborative International Dictionary of English v.0.48:

Aventurine \A*ven"tu*rine\, n. [F. aventurine: cf. It.
   avventurino.]
   1. A kind of glass, containing gold-colored spangles. It was
      produced in the first place by the accidental (par
      aventure) dropping of some brass filings into a pot of
      melted glass.
      [1913 Webster]

   2. (Min.) A variety of translucent quartz, spangled
      throughout with scales of yellow mica.
      [1913 Webster]

   Aventurine feldspar, a variety of oligoclase with internal
      firelike reflections due to the presence of minute
      crystals, probably of hematite; sunstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aventurine
    n 1: a translucent quartz spangled with bits of mica or other
         minerals [syn: sunstone, aventurine]