Pearl

Pearl is a biogenic gem material whose nacreous layer consists of about 95 percent calcium carbonate and 5 percent organic matrix. It forms in the bodies, or mantle tissue, of certain mollusks, usually around a microscopic irritant and always without human help in the natural case. This article treats the composition and structure of pearl, the distinction between natural and cultured formation, the principal cultured types, the factors that determine value, and the history and cultural heritage of pearling.

The nacreous layer of a pearl is a composite material whose mineral phase is dominated by aragonite, the same calcium carbonate polymorph that gives nacre its characteristic architecture. At the nanoscale each structural brick is itself a composite of aragonite mineral incorporating organic fibres, while the mortar between the bricks is a composite of chitin crystals embedded in a proteinaceous matrix. The organic matrix thus supplies both the framework and the chemical environment in which mineral growth is initiated and controlled. The resulting tablets are mostly hexagonal, about 0.5 micrometres thick and 3 to 5 micrometres across.

The arrangement of these tablets differs between molluscan groups. In bivalves nacre has the structure of a brick wall, with the tablets in each layer offset relative to the layers above and below, whereas in gastropods the tablets are piled one on top of another.

Nacre possesses a complex hierarchical microarchitecture that includes columnar architectures and sheet tiles, mineral bridges, polygonal nanograins, nanoasperities, plastic microbuckling, crack deflection and interlocking bricks. This architecture gives nacre a fracture toughness far exceeding that of monolithic aragonite, together with a marked capacity for energy absorption. In molluscan shells the nacreous layer sits within a three-layer structure comprising the periostracum, a prismatic calcite layer and an inner nacreous aragonite layer.

The formation of nacre proceeds through four steps: assembly of the matrix, initial formation of the mineral phase, nucleation of individual aragonite tablets, and growth into mature aragonite crystals. Mantle epithelial cells absorb calcium and carbonate from the extrapallial fluid into vesicles, forming amorphous calcium carbonate granules. A basic tenet in biomineralization is that biominerals grow by accretion of amorphous particles, which are later transformed into the corresponding mineral phase.

For Tahitian cultured pearls the thickness of this deposited nacre can be assessed free of charge upon registration for producers, and on request for a fee under certain conditions.

Natural pearls are a rarity of biology rather than manufacture: in the wild fewer than one in a thousand oysters produce a pearl during their lifetime. The earliest recorded mention of pearls is dated 2206 BC when a Chinese historian wrote about them for the first time.

Culturing begins with human intervention. In 1893 five semi-circular cultured pearls were harvested, and in 1905 Mikimoto succeeded in culturing perfect round pearls. Kokichi Mikimoto was born in 1858 and began his pearl culturing work in 1888 at the age of thirty. In 1927 during a tour of the United States and Europe Kokichi met Thomas Edison, who remarked that it was one of the wonders of the world that he was able to culture pearls.

The cultured pearl industry reshaped the trade in ways that reached far beyond Japan. In 1921 an article was published in a London newspaper claiming that cultured pearls sold by a Japanese merchant were only imitations, and the attack developed into a lawsuit known as the Paris trial, which Mikimoto eventually won in a French court. Bahrain's pearling activity suffered a sudden and catastrophic demise in the 1930s as a result of the development in Japan of cultured pearls from freshwater mussels.

These four are the major types of cultured whole pearls.

The qualities that determine the overall value of a natural or cultured pearl, or a piece of pearl jewellery, are size, shape, color, luster, surface quality, nacre quality and, for multi-pearl pieces, matching. Larger pearls are rarer and more valuable when other factors are equal. Round is the most difficult shape to culture, which makes it the rarest cultured pearl shape and generally the most valuable when other factors are equal.

Color comprises bodycolor, overtone and orient. Of the seven pearl value factors, luster might be the most important, and GIA grades it from Excellent, where reflections appear bright and sharp, down to Poor, where reflections are dim and diffused. Thin nacre is indicated by a visible nucleus or a dull chalky appearance, and it affects both luster and durability.

GIA gemologists begin by examining the pearl's surface to determine whether it is nacreous, showing layers of glowing nacre composed of calcium carbonate and protein, or non-nacreous, like the porcelaneous surfaces of conch or melo pearls.

X-ray radiography penetrates the pearl to reveal its internal structure, showing whether it is natural, bead cultured or non-bead cultured. Natural pearls typically display concentric ring structures with no solid nucleus, while bead cultured pearls reveal a central core and surrounding nacre. Trace elements help separate the types: manganese is typically higher in freshwater pearls and strontium in saltwater pearls, and reports may add trade terms such as hanadama or peacock for pearls of exceptional quality, rarity and market desirability.

Divers reached depths of up to eight fathoms, over fourteen metres, without breathing apparatus, remaining under water for typically up to a minute with cotton wool in the ears and a sponge over the mouth. They descended holding a weight such as a heavy stone and scraped oysters with a sharp knife into bags fastened round their waists. The pearl diver's weight held in the National Museum of Qatar is made of haematite, known as lasif, measuring 150 by 124 by 145.3 millimetres, attached to a rope, with the diver hooking his foot through a loop or twisting the rope around his leg to descend quickly.

Divers were generally known as jazwa and were ranked by profit shares. The nakhuda was the boat captain who took one fifth of total profits, while the tajir was the land captain who financed the sea nakhuda. Under the amil system the tajir financed the nakhuda without interest in return for the option of buying all the pearls in the catch. Merchants could not tell the quality or quantity of pearls inside the oysters they bought, and pearl merchants known as tawawish forwarded advances to captains, so the industry functioned on borrowed capital.

Pearl diving in the Gulf was a seasonal activity taking place over the four months of summer, with boats departing from ports such as Manama, Doha, Dubai and Abu Dhabi for coastal banks rich with oysters. In his 1838 book Travels in Arabia James Wellsted estimated that there were 3500 boats of all shapes and sizes at Bahrain at the height of the season, and a further 700 on the coast between Qatar and Oman. Lewis Pelly wrote a report on the pearling industry in 1865 stating that there were 1500 pearling boats active in Bahrain during the pearling season, yielding a profit of £400,000 each year.

The drop in global demand for pearls during the First World War might have been a short-term problem, but the numbers of good-sized pearls being harvested from the pearl beds of the Gulf were dwindling.

In 1931 the Glasgow manufacturer Tritonia Limited made enquiries about using its deep-sea diving suit to harvest pearls in the Gulf, but British Government officials rejected the request.

The Bahrain pearling property consists of seventeen buildings in Muharraq City, three offshore oyster beds, part of the seashore and the Qal'at Bu Mahir fortress on the southern tip of Muharraq Island. The Bū Māhir seashore and the individual sites in Muharraq have national protection as designated national monuments under decree law number 11 of 1995 concerning the protection of antiquities. The site is the last remaining complete example of the cultural tradition of pearling and the wealth it generated at a time when the trade dominated the Gulf economy from the 2nd century to the 1930s.

The walled coastal town of Al Zubarah flourished as a pearling and trading centre in the late 18th and early 19th centuries, before it was destroyed in 1811 and abandoned in the early 1900s.

The World Heritage Committee inscribed "Pearling, testimony of an island economy" in Bahrain on the World Heritage List in 2012 on the basis of criterion (iii).

La Peregrina is a pear-shaped pearl that originally weighed 223.8 grains, a figure reduced to 203.84 grains after drilling and cleaning in 1913. It was found by an African slave in the Pearl Islands in 1513 and turned over to Don Pedro de Temez, the administrator of Panama, who later gifted it to King Philip II of Spain. The Hamilton family owned La Peregrina until 1969 when it was sold at auction at Sotheby's in London and Richard Burton purchased it for $37,000 as a Valentine's Day gift for Elizabeth Taylor. In December 2011 the pearl sold for a record price of more than $11 million as part of Elizabeth Taylor's collection auctioned at Christie's in New York.

In French Polynesia Law 2017-16 of 18 July 2017 regulates professional activities related to production and commercialisation of pearl and nacre products. A global production quota is defined along with individual quotas proportional to farm size and weighted by producer- and lagoon-specific variables.

In 1917 Pierre Cartier traded a double strand of natural pearls for a mansion on Fifth Avenue in New York City.

Native Americans of the Atlantic coastal areas and the Mississippi River Basin were the first to collect and use United States freshwater mussel pearls and shells. The mother of pearl industry and the bead nuclei for the Japanese cultured pearl industry have supported the United States freshwater mussel fishery since the 1950s. In 1963 John Latendresse established the first experimental United States freshwater cultured pearl farm in Tennessee.

• 2206 BC
  Earliest recorded mention
  A Chinese historian writes about pearls for the first time.^{4Pearl Gemstone · Gemological Institute of America (GIA) — Gem Encyclopedia ↗}
• 1893
  First semi-circular harvest
  Five semi-circular cultured pearls are harvested.^{9Corporate Philosophy and History · Mikimoto Pharmaceutical Co., Ltd. ↗}
• 1905
  Perfect round pearls cultured
  Mikimoto succeeds in culturing perfect round pearls.^{9Corporate Philosophy and History · Mikimoto Pharmaceutical Co., Ltd. ↗}
• 1921
  London article and the Paris trial
  A London newspaper publishes an article claiming that cultured pearls sold by a Japanese merchant are only imitations; the attack develops into a lawsuit known as the Paris trial, which Mikimoto eventually wins in a French court.^{8Our Story — Kokichi Mikimoto · Mikimoto (K. Mikimoto & Co.) ↗}