However, a common point of confusion exists in the market. Much of the black stone sold as "Pure Onyx" in jewelry is, geologically speaking, black agate that has been dyed or treated. Natural, untreated black onyx is relatively rare. True Pure Onyx is distinct from agate only in the straightness of its bands; while agate has curved or concentric bands, onyx has parallel ones. This structure gives the stone a unique play of light and shadow, creating a sense of depth that solid black glass or plastic imitations cannot replicate.