Cat’s-Eye is an optical phenomenon (also known as chatoyancy) whereby light reflects back off a gemstone, cut en cabochon, as a single ray. Extremely fine parallel inclusions in the gemstone cause a silvery ray to appear perpendicular to the orientation of the inclusions.
Cat’s-Eye is also often used to refer to the cat’s-eye variety of Chrysoberyl.

Antique Cat’s-Eye Chrysoberyl and Diamond Ring.