A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters a bivalve mollusk, and settles inside the shell. The mollusk, being irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl. The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the mollusk's body and these small particles or organisms gain entry when the shell valves are open for feeding or respiration.
The ideal pearl is perfectly round and smooth, but many other shapes of pearls (baroque pearls) occur. A "natural pearl" or "wild pearl" is one that forms without any human intervention at all, in the wild, and is very rare. Many hundreds of pearl oysters or mussels have to be gathered and opened in order to find even one wild pearl, and for many centuries that was the only way pearls were obtained and the reason that pearls fetched such extraordinary prices in the past. Natural pearls are nearly 100% calcium carbonate and conchiolin. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.
Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those that are currently sold and these parls are formed in a pearl farm, using human intervention as well as natural processes. In cultured pearls, the irritant placed in the oyster is typically a piece of the mantle epithelium, with or without a spherical bead, thereby known as beaded or non-beaded cultured pearls. Trade names for cultured pearls are Akoya, South Sea, and Black Tahitian.
Salt water pearls can grow in several species of marine pearl oysters and live in oceans and are usually cultivated in protected lagoons or volcanic atolls.
Freshwater pearls are from freshwater pearl mussels that live in lakes, rivers, ponds and other bodies of fresh water. They can be found not only in hot climates, but also in colder climates in countries such as Scotland (where they are protected under law). However, most freshwater cultured pearls sold today come from China. Up to 40 pearls can be grown at once in one mussel and for a long time there was much emphasis on producing large quantities, but today science has improved the farming techniques resulting in higher quality freshwater pearls. It takes 3 to 7 years for the pearls to grow and their size ranges from 3-13 millimeters (average size being 6mm). Freshwater pearls come in a variety of pastel shades including white, pink, peach, lavender, grey, yellow and cream.
A popular aesthetic look for keshi pearls is to string together up to 50 strands of small pearls to form a substantial necklace. This look has been associated with fairly high class, and has stood the test of time enduring since the nineteen eighties. The price of keshi pearls is within the same realm as that of seed pearls. The harvesting and processing of these pearls is labor intensive.
Nucleated Freshwater Pearls that once existed in rumors were out in quantities for the world to see. They were not the round perfect spheres that pearl would expected but baroque pearls with tails. Unusual creatures but some were quite exceptional. It is very likely that bead nucleated freshwater pearls will be commercially produced in three years . The cultivation is in the trial stage at many farms with higher mortality rates and a high proportion of pearls with tails. If history is any indication, bead nucleated freshwater pearls will add a new chapter to the pearl world.
Imitation pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor, and generally speaking, artificial pearls are easily distinguished from genuine pearls. Some imitation pearls are simply made of mother-of-pearl, coral or conch shell, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient. Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly.Akoya Pearls
The diagram below shows a comparison of a cross-section of a cultured pearl in the upper diagram and a natural pearl in the lower diagram.
A well equipped gem testing laboratory is able to distinguish natural pearls from cultured pearls by using a gemological x-ray in order to examine the center of a pearl. With an x-ray it is possible to see the growth rings of the pearl, where the layers of calcium carbonate are separated by thin layers of conchiolin. The differentiation of natural pearls from non-beaded cultured pearls can be very difficult without the use of this x-ray technique.
Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing for imitations is to rub two pearls against each other. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, making both feel slightly gritty.