from kruos, "ice," and hence properly anything congealed and transparent, denotes "crystal," a kind of precious stone, Revelation 4:6; 22:1 . Rock crystal is pure quartz; it crystallizes in hexagonal prisms, each with a pyramidical apex.
"to be of crystalline brightness and transparency, to shine like crystal," is found in Revelation 21:11 , where it is said of Christ as the "Light-giver" (phoster) of the Heavenly City (not phos, "light," RV and AV). Possibly there the verb has a transitive force, "to transform into crystal splendor," as of the effect of Christ upon His saints.