The Collaborative International Dictionary of English v.0.48:

Euclase \Eu"clase\n. [Gr. e'y^ well, easily + ? to break. Cf. F.
   euclase, G. euklas. See named from its brittleness.] (Min.)
   A brittle gem occurring in light green, transparent crystals,
   affording a brilliant clinodiagonal cleavage. It is a
   silicate of alumina and glucina.
   [1913 Webster]