Easton's Bible Dictionary (1897)/Crystal
Crystal. (Ezek. 1:22, with the epithet "terrible," as dazzling the spectators with its brightness). The word occurs in Rev. 4:6; 21:11; 22:1. It is a stone of the flint order, the most refined kind of quartz. The Greek word here used means also literally "ice." The ancients regarded the crystal as only pure water congealed into extreme hardness by great length of time.