One way to hold this phrase: picture a piece of clean crystal, or a glass of still water. There is nothing inside it and nothing on it, so light goes straight through and whatever is on the far side arrives whole. English keeps that picture and moves it from place to place. Move it to a screen and every small detail survives the trip to your eye. Move it to a phone line and every word survives the trip to your ear. Move it to what somebody says and there is nothing in the way of the meaning either, so only one reading is left.
From the word’s history
The supplied Wiktionary entry traces the phrase to Middle English cristal cleer and calls it an emphatic form of clear, used both of what you can see through and of what you can understand. That fits the core image directly: crystal is named because it is the standard for letting light through, and the phrase borrows that standard for everything else.
封包裡的 Wiktionary 資料說這個說法可以追到中古英語的 cristal cleer，是 clear 的加強版，一開始就同時用在「看得穿」和「聽得懂」兩邊。這正好對上核心畫面：水晶被拿來當「清」的標準，後面所有用法都是借用同一個標準。
Wiktionary, "crystal clear"
登入就可以看每一支怎麼連回核心畫面。