What is the X in X-mas?
Old English cristal "clear ice, clear mineral," from Old French cristal (12c., Modern French crystal), from Latin crystallus "crystal, ice," from Greek krystallos, from kryos "frost," from PIE root *kru(s)- "hard, hard outer surface" (see crust). Spelling adopted the Latin form 15c.-17c. The mineral has been so-called since Old English; it was regarded by the ancients as a sort of fossilized ice. As a shortened form of crystal-glass it dates from 1590s. As an adjective, from late 14c.
crystal crys·tal (krĭs'təl)
A homogenous solid formed by a repeating, three-dimensional pattern of atoms, ions, or molecules and having fixed distances between constituent parts.
A mineral, especially a transparent form of quartz that has a crystalline structure and is often characterized by external planar faces.
Narcotics in powdered form, esp amphetamines; speed (1960s+ Narcotics)
(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.