出典:Wiktionary
Emphatic form of clear in visual and metaphorical “understandable” senses. Compare clear as a bell (“very clear (of sound)”).
In sense “understanding”, used both of explanations (by speaker) – “Her explanation of integration was crystal clear,” – and of understanding (by listener) “Is this crystal clear in that thick head of yours?”
液晶.
crystal clear skies
the crystalline lens
a crystal
結晶質なる
水晶澄みに澄んでいる
The water is clear as crystal―crystal clear.
the simplicity of a crystal