Which gemstone is known for its amazing optical effect called chatoyancy?

Chatoyancy, also known as the "cat's eye" effect, is a unique optical phenomenon seen in certain gemstones. It occurs when a specific type of inclusions or structures within the stone reflect light in such a way that a thin, bright band appears to move across the surface of the stone as it is turned.

Cat's-eye chrysoberyl is recognized for exhibiting this striking chatoyancy effect. When light hits the internal structure of cat's-eye chrysoberyl, the result is a distinct and captivating band of light that resembles the slit-like pupil of a cat’s eye. This effect is highly prized among gem collectors and contributes significantly to the gemstone's value and appeal.

In contrast, other gemstones like sapphire, ruby, and tourmaline may not exhibit this optical effect to the same extent, or at all, depending on their internal structure. While some sapphires can show asterism (a star-like effect), the singular characteristic of chatoyancy is particularly distinctive to cat's-eye chrysoberyl. Thus, the choice accurately highlights the gemstone that is most famous for this remarkable optical trait.