An optical effect where a gemstone shows a bright line of light that moves across its surface, like the eye of a cat (also called cat's eye effect).
From French chatoyant, derived from 'chaton' (cat's eye) with the root 'chat' (cat). The -ancy suffix creates an abstract noun. The effect got this name because the moving light line resembles a cat's eye.
Gemologists use this French word because it perfectly captures something almost supernatural—when you rotate a stone, a bright band follows you like a cat watching your every move. Language borrowed from nature's optical magic.
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