The Collaborative International Dictionary of English v.0.48:

Peridotite \Per"i*do*tite\, n. [Cf. F. p['e]ridotite.] (Min.)
   An eruptive rock characterized by the presence of chrysolite
   (peridot). It also usually contains pyroxene, enstatite,
   chromite, etc. It is often altered to serpentine.
   [1913 Webster]

   Note: The chief diamond deposits in South Africa occur in a
         more or less altered peridotite.
         [1913 Webster]

WordNet (r) 3.0 (2006):

peridotite
    n 1: a dark coarse-grained igneous rock consisting principally
         of olivine