From WordNet (r) 2.0 (August 2003) [wn]:
hiddenite
n : a green transparent form of the mineral spodumene used as a
gemstone
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Hiddenite \Hid"den*ite\, n. [After W. E. Hidden.] (Min.)
An emerald-green variety of spodumene found in North
Carolina; lithia emerald, -- used as a gem.
[1913 Webster]