The Collaborative International Dictionary of English v.0.48:

Idiomorphous \Id`i*o*morph"ous\, a. [Gr. 'idio`morfos of
   peculiar form; 'i`dios peculiar + morfh` form.]
   1. Having a form of its own.
      [1913 Webster]

   2. (Crystallog.) Apperaing in distinct crystals; -- said of
      the mineral constituents of a rock.
      [1913 Webster]