The complex structure of a cat's eye not only enables it to see at night but also gives it the appearance of glowing in the dark. Not surprisingly, jewels that sport a healthy luster are often compared with the feline ocular organ, so much that the term cat's-eye is used to refer to those gems (such as chalcedony) that give off iridescence from within. If you've brushed up on your French lately, you might notice that the French word for cat (chat) provides the first four letters of chatoyant, a word used by jewelers to describe such lustrous gems (and by others who see the same luster elsewhere). Chatoyant derives from the present participle of chatoyer, a French verb that literally means "to shine like a cat's eyes."
