This is the "cat's eye" effect that is seen in chrysoberyl, for example, here an eye can be seen when the stone is moved under a strong light. This is caused by the narrow inclusions in the stone.

The ability of certain gems to display a "cat's eye" effect, caused by the precise occurrence of narrow inclusions.

Chatoyancy or the Cat's Eye Effect is a reflection effect that appears as a single bright band of light across the surface of a gemstone. This phenomenon is commonly found in Chrysoberyl, Tourmaline and Tigers Eye.

(chatoyant) - having a changeable luster, like a cat's eye.

Phenomenon of certain cat's eye minerals which causes it to exhibit a concentrated narrow band of reflected light across the center of the mineral. Chatoyancy is usually only seen on polished cabochons. Chatoyant is the ability to exhibit chatoyancy.

The phenomenon of a sharp, well defined band or streak of light across the dome of a gem material caused by the reflection of light from parallel needle-like inclusions.

Cat's-eye-like effect on the surface of a cabochon stone.

The ability of certain gems to display a “cat's-eye” effect, due to the precise occurrence of narrow inclusions. These gems are always cut en cabochon.

Also know as the cat's eye effect, this is a reflection effect that appears as a single bright band of light across the surface of a gemstone, similar to the slit eye of a cat. It is caused by the reflection of light by parallel fibers, needles or channels. One the most coveted cat's eye is Cat's Eye Chrysoberyl - so much so, that if you just says cat's eye, it is assumed to be in reference to Cat's Eye Chrysoberyl. All other cat's eye gems, such as those found in Tourmaline or Tigers Eye, typically have an additional designation.

Chatoyancy is the appearance of a shimmering light that moves across the surface of certain stones and appears as a narrow line similar to a cat’s eye. This effect is most noticeable on a cabochon cut stone.

The lustrous, cat’s eye effect seen in some cabochon stones, like tiger’s eye, and sometimes in other stones, like aquamarine.

In gemology, chatoyancy (or chatoyance) is an optical reflectance effect seen in certain gemstones. Coined from the French, meaning "cat's eye," chatoyancy arises either from the fibrous structure of a material, as in tiger eye quartz, or from fibrous inclusions or cavities within the stone, as in cat's eye chrysoberyl. The effect can be likened to the sheen off a spool of silk; the mobile, wavering reflection always being perpendicular to the direction of the fibres.