Chatoyancy (n.) – The state or art of being chatoyant. Clink to hear pronunciation . First know use was in 1894.
Chatoyant (adj.) - having a changeable luster or color with an undulating narrow band of white light <a chatoyant gem> First known use of the word was in 1816.
French, from present participle of chatoyer to shine like a cat's eyes.
I think of it as the fire in an opal or a perfectly cut piece of labradorite. The sun picks up on the fire and it shines with brilliance or rather chatoyancy.