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The effect in certain translucent stones, when cut en cabochon and viewed in a suitable direction and light, of showing a streak of light in the interior that undulates across the curved surface as the position of the stone is changed.
It is caused by bundles of interior cavities or parallel needle-shaped crystals or to fibrous inclusions. When the stone is cut en cabochon and the base is set parallel to the length of the needles, the dome serves as a convex lens and each needle is shown as a point at the apex; together they form the gleaming streak. The result is a cats-eye effect found mainly in chrysoberyl and certain quartz, but also in other fibrous stones. Other stones showing chatoyancy are the tourmaline, diopside, andaluste, scapolite, and fibrolite.
A stone showing chatoyancy is sometimes said to be 'chatoyant'; the terms are derived from the French chat (cat).
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson