Matter whose structure repeats in perfect order, so its shape shows its nature. A lesson compressed until it can be carried.
Greek κρύσταλλος (krystallos, clear ice, crystal), from κρύος (kryos, frost, icy cold); matter whose structure repeats in perfect order, so its outward shape already reveals its inward nature.
Greek kryos is frost, and a crystal was first simply cold made solid and clear, its perfect repetition owed to the shape ice takes when nothing disturbs it while it forms. A lesson held this way keeps its order for the same reason: nothing was allowed to disturb it on the way to being carried.