phenakite
noun
phen·a·kite ˈfe-nə-ˌkīt 
ˈfē-
variants or phenacite
ˈfe-nə-ˌsīt
ˈfē-
: a hard glassy mineral that consists of a silicate of beryllium and occurs especially in rhombohedral crystals
Word History
Etymology
German Phenakit, from Greek phenak-, phenax deceiver; from its being easily mistaken for quartz
First Known Use
circa 1834, in the meaning defined above