What is a cat’s-eye effect and in which stones is it commonly observed?

• A. A chatoyancy effect where a bright band crosses the stone; chrysoberyl and some tourmalines
• B. Asterism effect seen in ruby and sapphire
• C. An iridescent sheen seen in opal
• D. A uniform luster change across the surface

Answer: (A)

The cat’s-eye effect is a form of chatoyancy—light reflects off parallel inclusions or fibers inside a gemstone to produce a single bright band that seems to glide across the surface as the stone or light moves. Chrysoberyl is the classic example where a fine, aligned rutile needle network creates a very sharp, movable band. Some tourmalines can also show this effect when they possess similar parallel inclusions. For maximum effect, stones are usually cut en cabochon with a smooth dome, so the band runs across the surface rather than along a facet. This phenomenon differs from asterism, which shows a star pattern, and from opal’s play-of-color, which is rainbow-like diffraction rather than a single Band.