* Crystals: Crystals, due to their highly ordered internal structure, are often exceptionally transparent and free from imperfections. This allows light to pass through them with minimal scattering, resulting in a very clear view.
* Metaphorical Extension: This physical property of crystals, their clarity, became a metaphor for mental clarity and understanding.
* Early Usage: The phrase started appearing in English literature and speech in the 18th century, reflecting this connection between the physical clarity of crystals and the mental clarity of thought.
Here are some examples of early usage:
* 1749: "The day was serene and clear, the sun shone bright, and the air was as crystal clear as the waters of a mountain stream." (From a letter by a Scottish writer)
* 1841: "The sky was crystal clear and the stars shone with a brilliance that was almost painful to the eye." (From a novel by Charles Dickens)
So, the phrase "crystal clear" is a powerful metaphor that draws on the visual clarity of crystals to communicate a state of mental or perceptual clarity.