Pearls come in two primary types: natural pearls that form unaided by humans and cultured pearls that humans cultivate with care. Cultured pearls fall into four main groups based on the type of water body and location. These are the saltwater Akoya pearls from Japan and China, the saltwater Tahitian pearls from French Polynesia, the saltwater South Sea pearls from Australia, Indonesia and the Philippines, and the freshwater pearls cultured in lakes and ponds in the United States and China.
A pearl forms when a mollusk reacts to an irritant inside its shell and starts creating layers of nacre. Lustrous pearls are stunning in every hue imaginable, and they are popular choices for jewelry items, as they symbolize both beauty and purity. They are graded according to their size, shape, color, luster, nacre quality and the surface. The smoother and more unblemished a pearl's surface, the higher its value. Thus, many substandard pearls may also undergo treatments to improve these traits and to increase value. In addition, there are imitation pearls; glass beads coated to look like real pearls but lacking the depth of luster of a real pearl. Typically, gemologists and jewelers can easily differentiate real and fake pearls.