Editing Pearl (section)

==Formation==

[[File:PearlCrossSection Figure1.pdf|thumb|upright=2.0|right|Formation of non–bead-cultured Akoya “keshi” pearls produced in a P. i. fucata mollusk. (A) Optical overview of a nonbeaded keshi–cultured pearl. (B) Cross-section showing CaCO3 growth begins onto an organic center. (C) Mature nacre. (D and E) Atomic-resolution image of atoms in nacre. (F) Transition from spherulitic aragonite structures to nacre. (G and H) Aggregated nanoparticles form massive aragonite. (I) The formation of nacre begins directly on massive aragonite. CC-license, PNAS 2021 118 (42); https://doi.org/10.1073/pnas.2107477118]]

The mollusk's mantle (protective membrane) deposits layers of [[calcium carbonate]] (CaCO<sub>3</sub>) in the form of the [[mineral]] [[aragonite]] or a mixture of aragonite and calcite (polymorphs with the same chemical formula, but different crystal structures) held together by an organic horn-like compound called [[conchiolin]]. The combination of aragonite and conchiolin is called [[nacre]], which makes up mother-of-pearl. The commonly held belief that a grain of sand acts as the irritant is rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the mollusk's body. These small particles or organisms gain entry when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically an introduced piece of the mantle epithelium, with or without a spherical bead (beaded or beadless cultured pearls).<ref>Neil H. Landman, et al. (2001) ''Pearls: A Natural History'', Harry Abrams, Inc., {{ISBN|0-8109-4495-2}}</ref><ref>{{cite web|url=http://www.fao.org/docrep/field/003/AB726E/AB726E11.htm |archive-url=https://web.archive.org/web/20080320144812/http://www.fao.org/docrep/field/003/AB726E/AB726E11.htm |archive-date=March 20, 2008 |title=Pearl oyster farming and pearl culture |publisher=Fao.org |access-date=May 17, 2013}}</ref>

===Natural pearls===
Natural pearls are nearly 100% calcium carbonate and [[conchiolin]]. 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, 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. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.

Typically, the build-up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting of nacre (tabular aragonite). In a pearl cross-section such as the diagram, these two different materials can be seen. The presence of columnar calcium carbonate rich in organic material indicates juvenile mantle tissue that formed during the early stage of pearl development. Displaced living cells with a well-defined task may continue to perform their function in their new location, often resulting in a [[cyst]]. Such displacement may occur via an injury. The fragile rim of the shell is exposed and is prone to damage and injury. Crabs, other predators and parasites such as worm larvae may produce traumatic attacks and cause injuries in which some external mantle tissue cells are disconnected from their layer. Embedded in the conjunctive tissue of the mantle, these cells may survive and form a small pocket in which they continue to secrete calcium carbonate, their natural product. The pocket is called a pearl sac and grows with time by cell division. The juvenile mantle tissue cells, according to their stage of growth, secrete columnar calcium carbonate from the pearl sac's inner surface. In time, the pearl sac's external mantle cells proceed to the formation of tabular aragonite. When the transition to nacre secretion occurs, the brown pebble becomes covered with a nacreous coating. During this process, the pearl sac seems to travel into the shell; however, the sac actually stays in its original relative position in the mantle tissue while the shell itself grows. After a couple of years, a pearl forms, and the shell may be found by a lucky pearl fisher.<ref>[[Kenneth Scarratt]], The Pearl and the Dragon, Houlton; 1st edition (1999) {{ISBN|0-935681-07-8}}</ref>

===Cultured pearls===
{{Main|Cultured pearl}}
[[File:Pearls.jpg|thumb|left|Nuclei from [[Mikimoto Pearl Island]], [[Toba, Mie|Toba]], [[Japan]]]]

Cultured pearls are the response of the shell to a tissue implant. A tiny piece of mantle tissue (called a ''graft'') from a donor shell is transplanted into a recipient shell, causing a pearl sac to form into which the tissue precipitates calcium carbonate. There are a number of methods for producing cultured pearls: using freshwater or seawater shells, transplanting the graft into the mantle or into the gonad, and adding a spherical bead as a nucleus. Most saltwater cultured pearls are grown with beads. Trade names of cultured pearls are Akoya ({{lang|ja|阿古屋}}), white or golden South Sea, and black [[Tahitian pearl|Tahitian]]. Most beadless cultured pearls are mantle-grown in freshwater shells in China and are known as freshwater cultured pearls.

Cultured pearls can be distinguished from natural pearls by [[X-ray]] examination.<ref name=farn2013>{{cite book|first=Alexander E.|last=Farn|title=Pearls : Natural, Cultured and Imitation|place=Burlington|publisher=Elsevier Science|year=2013|isbn=9781483162737|pages=90–108}}</ref> 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.

When a cultured pearl with a bead nucleus is X-rayed, it reveals a different structure to that of a natural pearl. A beaded cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings. A beadless cultured pearl (whether of freshwater or saltwater origin) may show growth rings, but also a complex central cavity, witness of the first precipitation of the young pearl sac.<ref name=farn2013/>

===Imitation pearls===
{{main|Imitation pearl}}
Some imitation pearls (also called shell pearls) are simply made of [[Nacre|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''.

===Gemological identification===
A well-equipped [[Gemology#Institutes, laboratories, schools, and publications|gem testing laboratory]] can distinguish natural pearls from cultured pearls by using gemological [[X-ray]] equipment to examine the center of a pearl. With X-rays, 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 pearl]]s 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.

===Value of a natural pearl===
[[File:Diadème de limpératrice Eugénie (musée du Louvre) (7166066743).jpg|thumb|Pearl tiara of Empress [[Eugénie de Montijo|Eugénie]] (1853) featuring 212 natural pearls, [[Louvre]], Paris.]]
Fine quality natural pearls are very rare jewels. Their values are determined similarly to those of other precious gems, according to size, shape, color, quality of surface, orient, and luster.

Single natural pearls are often sold as collector's items or set as centerpieces in unique jewelry. Very few matched strands of natural pearls exist, and those that do often sell for hundreds of thousands of dollars. (In 1917, jeweler [[Pierre Cartier (jeweler)|Pierre Cartier]] purchased the Fifth Avenue mansion that is now the New York Cartier store in exchange for a matched double strand of natural pearls Cartier had been collecting for years; at the time, it was valued at US$1&nbsp;million.)<ref>Strack, Elisabeth. "Pearls". Ruhle-Diebener-Verlag, 2006, p. 38 {{ISBN|3981084802}}.</ref>

The introduction and advancement of cultured pearls hit the pearl industry hard. Pearl dealers publicly disputed the authenticity of these new cultured products and left many consumers uneasy and confused about their much lower prices. Essentially, the controversy damaged the images of both natural and cultured pearls. By the 1950s, when a significant number of women in developed countries could afford their own cultured pearl necklace, natural pearls were reduced to a small, exclusive niche in the pearl industry.

===Origin of a natural pearl===
[[Image:Mary Stuart Queen.jpg|thumb|''[[Mary, Queen of Scots]]'' wearing a rope of pearls.<ref>Fiona Lindsay Shen, ''Pearl: Nature's Perfect Gem'' (London: Reaktion Books, 2022), pp. 160–165: [[John Duncan Mackie]], [https://archive.org/details/ScottishHistoricalReview18/page/n117/mode/2up "Queen Mary's Jewels", ''Scottish Historical Review'', 18:70 (January 1921), p. 94]</ref>]]
Previously, natural pearls were found in many parts of the world. Present-day natural pearling is confined mostly to the [[Persian Gulf]], in seas off [[Bahrain]]. Australia also has one of the world's last remaining fleets of pearl diving ships. Australian pearl divers dive for South Sea pearl oysters to be used in the cultured South Sea pearl industry. The catch of pearl oysters is similar to the numbers of oysters taken during the natural pearl days. Hence, significant numbers of natural pearls from wild oysters are still found in the Australian Indian Ocean waters. X-ray examination is required to positively verify natural pearls found today.

===Types of cultured pearls===
{{main|Keshi pearls}}
[[File:Mabé.jpg|thumb|A blister pearl, a half-sphere, formed flush against the shell of the pearl oyster.]]

A [[keshi pearl]] is a pearl composed entirely of nacre and results from mishaps in the culturing process.  Most are quite small, typically only a few millimeters in diameter, and are often irregular in shape. In seeding a cultured pearl, a piece of mantle muscle from a sacrificed oyster is placed with a bead of mother of pearl within a host oyster. If the piece of mantle should slip off the bead, a keshi pearl forms of baroque shape about the mantle piece. Therefore, while a keshi pearl could be considered superior to cultured pearls with a mother-of-pearl bead center, in the cultured pearl industry, the oyster's resources used to create a mistaken all-nacre baroque pearl is a drain on the production of the intended round cultured pearl. Therefore, the pearl industry is making ongoing attempts to improve culturing techniques so that keshi pearls do not occur. All-nacre pearls may one day be limited to natural found pearls.<ref>The Pearl Book, pp. 47–49, 4th edition, Matlins, Antoinette, Gemstone Press, copyright 2008</ref><ref>The Pearl Story, Gemstone Adventure Series, volume 2, copyright 2007 America's Collectibles Network,</ref><ref>{{cite journal| last = Hanni| first=H A| title=Keshi Perlen: Ein Erklarungbedurftiger Begriff (Keshi Pearls: a term in need of explanation) | journal=Zeitschrift der Deutschen Gemmologischen Gesellschaft | volume=55 |issue=1–2| pages=39–50 |publisher=DGemG |date=June 2006 }}</ref> Today many "keshi" pearls are intentional, with post-harvest shells returned to the water to regenerate a pearl in the existing pearl sac.

[[Tahitian pearl]]s, frequently referred to as black pearls,<ref>{{cite journal | last1 = Pouvreau | first1 = Stéphane | display-authors = etal   | year = 2000| title = Ecophysiological model of growth and reproduction of the black pearl oyster, ''Pinctada margaritifera'': potential applications for pearl farming in French Polynesia | journal = Aquaculture | volume = 186 | issue = 1–2| pages = 117–144 | doi = 10.1016/S0044-8486(99)00373-7 | bibcode = 2000Aquac.186..117P | url = http://archimer.ifremer.fr/doc/2000/publication-380.pdf }}</ref> are highly valued because of their rarity; the culturing process for them dictates a smaller volume output and they can never be mass-produced because, in common with most sea pearls, the oyster can only be nucleated with one pearl at a time, while freshwater mussels are capable of multiple pearl implants. Before the days of cultured pearls, black pearls were rare and highly valued for the simple reason that white pearl oysters rarely produced naturally black pearls, and black pearl oysters rarely produced any natural pearls at all.

Since the development of pearl culture technology, the black pearl oysters ''Pinctada margaritifera'' found in [[Tahiti]] and many other Pacific islands, including the [[Cook Islands]] and [[Fiji]] are being extensively used for producing cultured pearls. 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 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 black cultured pearl. This is simply because the black pearl oyster ''Pinctada margaritifera'' is far more abundant than the elusive, rare, and larger south sea pearl oyster ''Pinctada maxima'', which cannot be found in lagoons, but which must be dived for in a rare number of deep ocean habitats or grown in hatcheries.

Natural black pearls are rare, with black pearls having a body color that may be assessed as silver, silver blue, gold, brown-black, green-black, or black.<ref>{{cite journal |last1=Goebel |first1=Marisa |last2=Dirlam |first2=Dona M. |title=Polynesian Black Pearls |journal=Gems & Gemology |date=Fall 1989 |volume=25 |issue=3 |pages=130–148|doi=10.5741/GEMS.25.3.130 |doi-access=free }}</ref>

Black cultured pearls from the black pearl oyster&nbsp;– ''Pinctada margaritifera''&nbsp;– are not South Sea pearls, although they are often mistakenly described as black South Sea pearls. In the absence of an official definition for the pearl from the black all use to, these pearls are usually referred to as "black pearls".{{Citation needed|date=December 2011}}

The correct definition of a South Sea pearl&nbsp;– as described by CIBJO and GIA&nbsp;– is a pearl produced by the ''Pinctada maxima''<ref>{{citation 
| last = Mamangkey | first =Noldy 
| date = 2009
| title=Improving the quality of pearls from Pinctada maxima
| place = PhD thesis
| publisher=James Cook University
| doi =10.25903/yfn1-4k54 
| url = https://researchonline.jcu.edu.au/11722/| type =phd 
}}</ref> pearl oyster. South Sea pearls are the color of their host ''Pinctada maxima'' oyster&nbsp;– 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.

South Sea pearls are the largest and rarest of the cultured pearls&nbsp;– making them the most valuable.<ref>{{Cite web|title=How To Buy Pearls|url=https://nature.berkeley.edu/classes/eps2/wisc/pbuy.html|access-date=2023-02-11|website=nature.berkeley.edu}}</ref><ref name="gia"/> Prized for their exquisitely beautiful 'orient' or lustre, South Sea pearls are now farmed in various parts of the world where the ''Pinctada maxima'' oysters can be found, with the finest South Sea pearls being produced by [[Paspaley]] along the remote coastline of North-Western Australia.<ref name="gia">{{cite journal |journal=Gems & Gemology |year=2012 |volume=48 |issue=4 |pages=236–261 |title=Natural Pearls from Australian ''Pinctada maxima'' |author=Scarratt, Kenneth  |doi=10.5741/GEMS.48.4.236 |s2cid=130970595 |doi-access=free }}</ref><ref>{{cite web|url=  http://www.leadingwomen.org/_FileLibrary/InTheNews/14/pasparley.pdf|title=  Jewel of the South Sea|url-status=  live|archive-url=  https://web.archive.org/web/20130930070019/http://www.leadingwomen.org/_FileLibrary/InTheNews/14/pasparley.pdf|archive-date=  September 30, 2013|df=  mdy-all}}</ref> White and silver colored South Sea pearls tend to come from the [[Broome, Western Australia|Broome]] area of Australia, while golden colored ones are more prevalent in the Philippines and Indonesia.

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.<ref>[http://www.perlas.com.mx/blog/tag/concha-nacar/ Concha Nacar | The Sea of Cortez Pearl Blog] {{webarchive|url=https://web.archive.org/web/20140225010229/http://www.perlas.com.mx/blog/tag/concha-nacar/ |date=February 25, 2014 }}. Perlas.com.mx (November 22, 2010). Retrieved on October 14, 2015.</ref>  Also called Concha Nácar, the pearls from these rainbow-lipped oysters fluoresce red under [[ultraviolet]] light.