Unlike gemstones such as diamonds, pearls are not judged by a single international standard of quality. Various systems have been developed for grading pearls on the basis of luster, color, size, shape, surface and iridescence. Jewelers sort pearls according to grade, grouping together gems of similar quality. And although mollusks do most of the work of making pearls, humans use a variety of techniques to improve the quality of pearls or to introduce effects not found in nature. Such methods range from simple washing and buffing to more radical treatments such as exposing pearls to radiation or faceting them. Despite these common "enhancements," the pearl industry still favors the high-quality, blemish-free spherical pearl as its most perfect form.