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.