What Is a Cat Eye Stone?
That single bright band of light moving across the stone has a name: chatoyancy. Here is what causes it and how to look at it.
The band of light
Turn a cat eye stone under a lamp and a single bright line travels across its surface, staying at right angles to the light. It looks like the slit pupil of a cat's eye, which is where the name comes from.
The proper word for the effect is chatoyancy, from the French for cat's eye. It describes an optical effect, not a material - which is why the trade uses it for several different stones.
What causes it
Chatoyancy comes from parallel fibres or channels running through the stone. Light entering the stone reflects off those fibres and is gathered into a single band at right angles to them.
The effect only shows if the stone is cut correctly. It has to be cut as a cabochon - domed and polished rather than faceted - and the dome has to be oriented so the fibres run across it. Cut it the wrong way and the effect vanishes entirely.
How to look at one
Hold it under a single light source. Two lights give two bands and the effect gets muddled, which is also why cat eye stones are difficult to photograph well.
Rock it slowly. A good cat eye has a sharp, narrow, bright band that moves cleanly. A weaker one has a broad, hazy band that barely shifts.
The band should sit across the middle of the dome. If it sits off to one side, the stone was cut slightly off-axis.
Why our rings use them
Every ring here is made by hand, and the effect suits what we make - large, domed stones set one at a time into heavy bezels, where a flat faceted stone would just look small.
It also means no two of our rings look the same in motion. Two stones can be the same color and still catch the light completely differently, which is the part that does not come across in a photograph.