Keshi pearls, although they often come by accident, are not considered natural. They are a byproduct of the growth process and therefore do not occur without human intervention. They are quite small, usually only a few millimeters. Keshi pearls are produced by many different types of marine shellfish and freshwater mussels in China. Keshi pearls are actually a bug in the cultured pearl planting process. When sowing the cultured pearl, a piece of muscle from the mantle of an oyster sacrificed with a pearl of nacre is placed inside the oyster. If the cloak piece slips off the string, a baroque-shaped pearl forms around the cloak piece that is completely nacre.
Therefore, a Keshi pearl could be considered superior to cultured pearls with a mother of pearl center. In the cultured pearl industry, the resources used to create the wrong mother of pearl baroque pearl are a drain for the production of round cultured pearls. Therefore, they are trying to improve the cultivation technique so that keshi pearls are not produced. All nacreous pearls may someday be limited to natural pearls.    Today, many “keshi” pearls are actually intentional, and the post-harvest shells are returned to the water to regenerate a pearl in the existing pearl sack.
Tahitian pearls, often called black pearls,  are highly prized due to their rarity; the cultivation process for them imposes a lower production volume and they can never be mass produced because, like most sea pearls, the oyster can only nuclear with one pearl at a time, while freshwater mussels are able to implant more pearls. Before the days of cultured pearls, black pearls were rare and highly prized for the simple reason that white pearl oysters rarely produced naturally black pearls, and black pearl oysters rarely produced natural pearls.
Since the development of pearl farming technology, the black pearl oysters Pinctada margaritifera found in Tahiti and many other Pacific islands, including the Cook Islands and Fiji, are used extensively to produce cultured pearls. The rarity of the cultured black pearl is now a “comparative” issue. Black cultured pearl is rare compared to Chinese freshwater cultured pearls and Japanese and Chinese akoya 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 cultured black pearl. This is simply because the black pearl oyster Pinctada margaritifera is much more abundant than the elusive, rare and larger southern sea pearl oyster Pinctada maxima, which cannot be found in lagoons but must be submerged in a few numbers, common to deep ocean habitats. or grown in hatcheries.
Black pearls are rarely black – they are usually shades of green, purple, eggplant, blue, gray, silver, or peacock (a mix of various shades, such as a peacock’s feather). [Citation required]
The cultured black pearls of the black pearl oyster, Pinctada margaritifera, are not South Sea pearls, although they are often mistakenly described as South Sea black pearls. In the absence of an official definition for the black oyster pearl, these pearls they are generally referred to as “black pearls”. [Citation required]
The correct definition of a South Sea pearl, as described by CIBJO and GIA, is a pearl produced by the pearl oyster Pinctada maxima . South Sea pearls are the color of the host Pinctada maxima oyster and can be white, silver, pink, gold, cream, and any combination of these base colors, including shades of the various colors of the rainbow shown in the mother-of-pearl of the ‘oyster shell itself.