greisen

from WordNet (r) 3.0 (2006)

greisen
    n 1: a granitic rock composed of quartz and mica

from The Collaborative International Dictionary of English v.0.48

Greisen \Grei"sen\, n. (Min.)
   A crystalline rock consisting of quarts and mica, common in
   the tin regions of Cornwall and Saxony.
   [1913 Webster]