Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

demantoid
     n : a green andradite used as a gemstone