Reference work entry
Chatoyancy is an optical reflectance effect on a curved gemstone surface that can be likened to the sheen of a spool of silk, and it looks like a cat’s eye. Chatoyancy arises from the fibrous structure or fibrous inclusions within a stone that are in parallel alignment. When the incoming light is reflected or scattered, a luminous streak of reflected light perpendicular to the direction of the fibres can be seen, like a cat’s eye. The band of light is sharp and moves across the stone in correspondence to the light intensity. This effect is called chatoyancy or the ‘cat’s eye effect’. Many gems, such as cat’s eye aquamarine, cat’s eye tourmaline and cat’s eye apatite, have this effect, and chrysoberyl has the most remarkable cat’s eye effect (Fig. 25).
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