The Collaborative International Dictionary of English v.0.48:

Demantoid \De*man"toid\, n. [G. demant diamond + -oid.] (Min.)
   A yellow-green, transparent variety of garnet found in the
   Urals. It is valued as a gem because of its brilliancy of
   luster, whence the name.
   [1913 Webster]

WordNet (r) 3.0 (2006):

demantoid
    n 1: a green andradite used as a gemstone