Cat's Eye Opal

Cat's eye opal is opal that displays chatoyancy, a single bright band of reflected light that glides across a domed cabochon like the slit pupil of a cat's eye. The effect is most often seen in common opal rather than play-of-color opal, in honey, golden, green and brownish tones.

The phenomenon is produced by parallel fibrous or tubular structures within the stone that reflect light into a focused line. To show the effect well, the rough must be oriented carefully and cut as a high cabochon. Much cat's eye opal comes from Tanzania, Madagascar and Brazil.

It should not be confused with chrysoberyl cat's eye (cymophane), the classic and much harder "cat's eye" gem of the trade.

Cat's eye opal forms like other common opal, by precipitation of hydrated silica from silica-rich solutions into cavities and seams in host rock. The chatoyancy arises when the opal develops an internal fabric of fine parallel fibers, channels or oriented inclusions as it solidifies.

When the stone is cut en cabochon with the dome perpendicular to these parallel structures, light reflecting off the aligned fibers concentrates into a single bright band. Honey and green Tanzanian cat's eye opal, for example, forms in weathered volcanic and sedimentary settings where silica gels hardened with a consistent fibrous orientation. The strength and sharpness of the eye depend on how fine and well-aligned the internal fibers are.

Look for a single sharp band of light that moves across a domed cabochon as it is rotated under a single light source, set in a translucent honey, yellow, green or brown body. Hardness is moderate (5.5-6.5), lower than chrysoberyl cat's eye (8.5), which is the key distinction from the premium "cat's eye" gem.

The luster is waxy to slightly silky, and the stone is relatively light in hand. Distinguish from cat's eye chrysoberyl, tourmaline and quartz by hardness and weight: opal is softer and less dense. A genuine eye is a crisp single line, not a diffuse glow; a vague sheen suggests poor orientation or an imitation.

Cat's eye opal is cut exclusively en cabochon to capture the moving line of light and is set in rings, pendants and earrings. It offers an affordable way to own a chatoyant gem compared with chrysoberyl cat's eye.

As an opal it is the October birthstone and shares opal's metaphysical reputation for creativity and emotional insight, with the "eye" sometimes marketed as offering protection and focus. Because it is softer and can be water-sensitive, it should be protected from scratches, knocks, heat and chemicals, and cleaned with only a soft damp cloth. Buyers should confirm whether a "cat's eye" gem is opal or the harder chrysoberyl, since value differs sharply.