The difference between natural and cultured pearls focuses on whether the pearl is created spontaneously by nature or not. Pearls are formed inside the shell of certain mollusks. As a prevention of a potentially threatening irritant, the mollusk creates a pearl to seal off the irritation.
The mantle of the mollusk deposits layers of calcium carbonate (CaCO3) in the form of the minerals argentite or a mixture of argentite and calcite (both crystalline forms of calcium carbonate) held together by an organic horn-like compound. The combination of argentite is called nacre, which makes up mother-of-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 animal body. These small particles or organisms enter the animal when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically a cut piece of the mantle epithelium, together with processed shell beads, the combination of which the animal accepts into its body.
Cultured pearls (nucleated and not nucleated or tissue nucleated cultured pearls) and imitation pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often ‘preformed’ as they tend to follow the shape of the implanted shell bead nucleus. Once the preformed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more.
When a nucleated cultured pearl is lit, it reveals a different structure to that of a natural pearl. A cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings.
Natural pearls are nearly 100% calcium carbonate. 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, secretes the calcium carbonate to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.
The rarity of the black cultured pearl is now a “comparative” issue. The black cultured pearl is rare when compared to Chinese freshwater cultured pearls, and Japanese and Chinese cultured pearls, and is more valuable than these pearls. However, it is more abundant than the South Sea pearl, which is more valuable than the black cultured pearl. This is simply because the black pearl oyster is far more abundant than the elusive, rare, and larger south sea pearl oyster, which cannot be found in lagoons, but which must be dived for in a rare number of deep ocean habitats or grown in hatcheries.