question: How do oysters make pearls?
comment: A sand granule (or any other small thing like a piece of shell or coral) gets inside the shell, lodging close to the oyster's body. It aggravates the oyster and to feel more comfortable the oyster starts coating the sand with spit which creates the round and smooth pearl.
passages: A farm in the Gulf of California, Mexico, is culturing pearls from the black lipped "Pinctada mazatlanica" oysters and the rainbow lipped "Pteria sterna" oysters. Also called Concha Nácar, the pearls from these rainbow lipped oysters fluoresce red under ultraviolet light.
  
  Section::::From other species.
   Cultured 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. After a bead is inserted into the oyster, it secretes a few layers of nacre around the bead; the resulting cultured pearl can then be harvested in as few as twelve to eighteen months.
   The two methods of culturing pearls insert either "seeds" or beads into oysters. The "seed" method uses grains of ground shell from freshwater mussels, and overharvesting for this purpose has endangered several freshwater mussel species in the southeastern United States. The pearl industry is so important in some areas, significant sums of money are spent on monitoring the health of farmed molluscs.
   Cultured pearl
  
  A cultured pearl is a pearl created by a mussel farmer or oyster farmer under controlled conditions. Cultured pearls can be farmed using two very different groups of bivalve mollusk: the freshwater river mussels, and the saltwater pearl oysters.
  
  Section::::Development of a pearl.
   Section::::Aquaculture.:Production techniques.:Broodstock.
   In nature, pearl oysters produce pearls by covering a minute invasive object with nacre. Over the years, the irritating object is covered with enough layers of nacre to become a pearl. The many different types, colours and shapes of pearls depend on the natural pigment of the nacre, and the shape of the original irritant.
   The various species of "Pinctada" produce different maximum sizes and colors of pearls, depending on the size of the species and the natural color of the nacre inside the shell. Black South Sea pearls, or Tahitian pearls come from the black-lip oyster; white and golden South Sea pearls from the white-lip and golden-lip oysters; and Akoya cultured pearls from "Pinctada fucata martensii", the Akoya pearl oyster.
  
  Pearls are also obtained in commercial quantities from some species of the closely related winged oyster genus "Pteria".
   The largest pearl-bearing oyster is the marine "Pinctada maxima", which is roughly the size of a dinner plate. Not all individual oysters produce pearls naturally. In fact, in a harvest of two and a half tons of oysters, only three to four oysters produce what commercial buyers consider to be absolute perfect pearls.
   Section::::Process.
   The correct definition of a South Sea pearl – as described by CIBJO and GIA – is a pearl produced by the "Pinctada maxima" pearl oyster. South Sea pearls are the color of their host "Pinctada maxima" oyster – and can be white, silver, pink, gold, cream, and any combination of these basic colors, including overtones of the various colors of the rainbow displayed in the pearl nacre of the oyster shell itself.
   Natural (or wild) pearls, formed without human intervention, are very rare. Many hundreds of pearl oysters or mussels must be gathered and opened, and thus killed, to find even one wild pearl; for many centuries, this was the only way pearls were obtained, and why pearls fetched such extraordinary prices in the past. Cultured pearls are formed in pearl farms, using human intervention as well as natural processes.
   Section::::Grafting.
   Section::::Pioneering cultured pearls.
  
  Mikimoto began his search of an alternative method to produce pearls as the chairman of the Shima Marine Products Improvement Association. At this point the demand for pearls had severely outweighed the supply, prompting the consideration of an effort to protect the oysters.
   The most popular and effective method for creating cultured pearls are made from the shells of freshwater river mussels harvested in the midwestern states of the U.S., from Canada to the Gulf of Mexico. Shells with the common names, "Washboard" "Maple Leaf" "Ebony" "Pimpleback" and "Three Ridge" are popular for use in pearl culture due to their compatibility with the host animal, and the nacre they are to be covered by. These high-quality and sought-after shells are first sliced into strips and then into cubes. The edges and corners are ground down until they are a roughly spherical and then milled to become perfectly round, and brought to a highly polished finish.
   Scallops do occasionally produce pearls, though scallop pearls do not have the buildup of translucent layers or "nacre" which give desirability to the pearls of the feather oysters, and usually lack both lustre and iridescence. They can be dull, small, and of varying colour, but exceptions occur that are appreciated for their aesthetic qualities.
  
  Section::::Biology.:Anatomy and physiology.:Mutualism.
   Section::::Biology.:Genetics.
  
  The genome of "M. gigas" has been sequenced, revealing an extensive set of genes that enables it to cope with environmental stresses.
  
  Section::::Aquaculture.
  
  Section::::Aquaculture.:Historical background.
   Section::::Aquaculture.:Production techniques.
  
  Numerous methods are used in the production of Pacific oysters. These techniques depend on factors such as the seed supply resources, the environmental conditions in the region, and the market product, i.e., whether the oysters are sold in a half shell, or shelled for meat extraction. Production can either be entirely sea-based or rely on hatcheries for seed supply.
  
  Section::::Aquaculture.:Production techniques.:Seed supply.
   This species is commonly farmed and harvested for pearls, and there is general consensus that the quality of pearls from "Pinctada margaritifera" is the highest quality out of all the pearl oysters. Pearls form when a small particle enters into the oyster and nacre is released by the oyster to coat the particle or object, eventually creating a small pearl. The particle might be a grain of sand, organic material, or even a parasite. The oyster’s release of the nacre serves as an adaptation of the immune system to isolate the invasive particle and irritation. "P. margaritifera" in particular produces gray or black pearls.
   Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable. The best natural pearls are produced by marine pearl oysters, "Pinctada margaritifera" and "Pinctada mertensi", which live in the tropical and subtropical waters of the Pacific Ocean. Natural pearls form when a small foreign object gets stuck between the mantle and shell.
   Section::::Pearls.
  
  This volute is known to produce pearls; however the "Melo melo" pearl has no nacre, unlike the pearl of a pearl oyster. The GIA and CIBJO now simply use the term 'pearl' (or, where appropriate, the more descriptive term 'non-nacreous pearl') when referring to such items, rather than the previously-used term 'calcareous concretion' and, under Federal Trade Commission rules, various mollusc pearls may be referred to as 'pearls' without any qualification. The melo pearl is created by the mollusc in the same way as other pearls are created by other molluscs. See also: Conch Pearl
  
  Section::::Human use.
   Wild oysters of this species seldom contain pearls. When they do, they tend to be irregular in shape and have the same range of pinkish hues that are typical of the nacre lining the shells. The maximum diameter of the pearls is about . Because the shell valves of this species are so thin, it was beyond the competence of nineteenth century oyster culturists to use it for pearl production.
   Section::::Culturing.
   Section::::History.
  
  Mikimoto Kōkichi was able to use Nishikawa's technology. After the patent was granted in 1916, the technology was immediately commercially applied to akoya pearl oysters in Japan in 1916. Mise's brother was the first to produce a commercial crop of pearls in the akoya oyster. Mitsubishi's Baron Iwasaki immediately applied the technology to the South Sea pearl oyster in 1917 in the Philippines, and later in Buton and Palau. Mitsubishi was the first to produce a cultured South Sea pearl – although it was not until 1928 that the first small commercial crop of pearls was successfully produced.
   Natural pearls are formed by nature, more or less by chance. On the other hand, cultured pearls are human creations formed by inserting a tissue graft from a donor mollusk, upon which a pearl sac forms, and the inner side precipitates calcium carbonate, in the form of nacre or "mother-of-pearl". 
   Section::::Biology.:Life cycle.
  