Adjective
(simile, of an object) Entirely transparent.
(simile, of a statement) Easily understandable.
Source: en.wiktionary.orgI'm full of fears and I do my best to avoid difficulties and any kind of complications. I like everything around me to be clear as crystal and completely calm. Alfred Hitchcock
Now, the redemption which we as yet await (continued Imlac), will be that of Kalki, who will come as a Silver Stallion: all evils and every sort of folly will perish at the coming of this Kalki: true righteousness will be restored, and the minds of men will be made as clear as crystal. James Branch Cabell