Tahitian pearl

A Tahitian pearl is a cultured pearl grown in the black-lipped pearl oyster (Pinctada margaritifera). Although named after Tahiti, most of the farms lie in the lagoons of the Tuamotu and Gambier archipelagos of French Polynesia. It is popularly called a “black pearl”, yet most are not jet black but charcoal grey, silver or dark green, with purple, pink and blue overtones. Usually 8–18 mm across, they are much larger than Akoya pearls. First cultured in the 1960s, they soon became French Polynesia’s most important export.

Jewellery: used in necklaces, earrings and rings; its large size and naturally dark colour set it apart from other cultured pearls. In most harvests more than 40 percent of the pearls come out baroque or semi-baroque (irregular in shape), and these are used in jewellery too. Mother-of-pearl: for centuries the oyster was fished for its shell; in the 19th century European sailors traded for the shells, which were used for buttons, inlay and jewellery. Economy: in 2004 pearls made up 71 percent of French Polynesia’s local exports by value and supported about 5,000 people.

It has a hardness of 2.5–3 on the Mohs scale. A fingernail cannot scratch it, but a steel knife can. Its specific gravity is 2.72–2.78 g/cm³.

Its best-known localities: Gambier Islands (Mangareva), French Polynesia; Manihiki Atoll, Cook Islands; Manihi Atoll, Tuamotu, French Polynesia; Ahe Atoll, Tuamotu, French Polynesia; Arutua Atoll, Tuamotu, French Polynesia.

Size is a clue: Tahitian pearls are usually 8–18 mm across, mostly 9–12 mm, and few are smaller than 9 mm. Very black, round pearls of 8 mm or less are generally dyed or irradiated Akoya pearls. The colour is not flat: as the pearl is turned, green, blue, purple and pink overtones play over a grey, black or brown body. The most characteristic colour is a greenish black known as “peacock”. A strand in which every pearl is exactly the same colour, or dye concentrated around the drill hole under a loupe, points to dyeing. Dyed pearls: since the early 1900s small, light-coloured Akoya pearls have been dyed black with silver nitrate solutions, and light-coloured Tahitian and South Sea pearls have also been found darkened. Under long-wave ultraviolet light natural-colour Tahitian pearls glow a dull reddish brown, while dyed pearls usually show no glow or a dull green one; a laboratory makes the final call.

On the “street to diamond” value ladder it sits on step 7 of nine: Precious stone. Colour, lustre, shape, size and surface quality matter most. The greenish-black “peacock” colour and aubergine (purple) overtones are the most sought after; silver and golden tones are worth less because other South Sea oysters produce them too. Perfectly round pearls are scarce: in a first harvest only about 5 percent are round. In 2002 French Polynesia set a minimum nacre thickness of 0.8 mm for exported pearls; treatments such as dyeing should be disclosed.

According to Tahitian legends, black pearls were emanations from the gods; the important god Oro was believed to have come down to earth on his rainbow, which was thought to be the source of the iridescence of the pearl and its shell. The colours and iridescence of a Tahitian pearl come not from a rainbow but from light being refracted and reflected by the very thin layers of aragonite and conchiolin that make up nacre, and from organic pigments called porphyrins in the oyster’s shell. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.