Natural pearls form in the ‘wild’ without human intervention. They are formed when an irritant becomes trapped within a mollusc’s mantle folds. The oyster or mussel, in an endeavour to relieve itself of the irritation, creates a pearl sac of external mantle tissue cells and secretes calcium carbonate and conchiolin (known as nacre) to cover the irritant. The secretion process is repeated many times, thus producing the pearl. Accordingly, a natural pearl is solid pearl.
Natural pearls have been highly valued from early time.
The Red Sea famously yielded pearls that often found their way to Egyptian pharaohs – indeed Cleopatra is reported to have dissolved a priceless natural pearl earring in a glass of wine before drinking the potion to impress Mark Antony.
The abundance of mother of pearl shell in the vicinity of Broome led to the township being established in the 1880s as a pearling centre. The primary target of the divers was the shells but very occasionally a natural pearl would be found in the oyster. Many hundreds of oysters would need to be opened (and therefore killed) to find one ‘wild’ pearl.
The invention of plastic almost completely eliminated the consumption of mother of pearl for buttons. The Depression impacted on demand for natural pearls and the Second World War brought an almost complete halt to pearling activities.
Ironically, some (very few) natural pearls are now being found again as a by-product of the resumption of diving for live oysters – this time for use in the cultured pearl farms in the north of Western Australia.
Seawater pearls include Akoya, South Sea and Tahitian pearls.
Akoya pearls are produced by a species of small pearl oyster (6–8 cm) in size; usually only one pearl is grown at a time. Their size rarely exceeds 10 mm. Commercial Akoya pearl production began in Japan around 1916. Despite common belief, Mikimoto did not discover the process of pearl culture, but its brand name is the most widely known of Akoya pearls.
Japan has almost completely ceased its production of Akoya pearls smaller than 8 mm, and in 2010 China became the largest producer.
South Sea cultured pearls are grown in larger oysters in the South Pacific and Indian Oceans – principally in Australia, Indonesia and the Philippines. South Sea pearls are characterised by their large size (typically 10–14 mm) and warm lustre. The colour of South Sea pearls corresponds to that of their host oyster and can be white, pink, gold, cream or silver.
Tahitian pearls, frequently referred to as black pearls, are formed in black pearl oysters found in Tahiti and many other Pacific Islands. They are not South Sea pearls. Although referred to as ‘black’, they are generally found in shades of green, purple, aubergine, blue, silver or ‘peacock’.
China is the largest producer of pearls cultured from freshwater mussels – annual production exceeds 1500 metric tons per year. These mussels can grow multiple cultured pearls.
In the 1970s and ’80s the most commonly produced Chinese freshwater pearls were known as ‘Rice Kriple’ – high volumes of low quality pearls. However, in recent decades production has shifted to a triangular shell mussel, producing few pearls per oyster, but of far higher quality.
Freshwater pearls are generally treated to enhance their appearance. Keshi pearls are found as a by-product of the culturing process, growing without a nucleus, typically quite small (only a few millimetres in size) and irregularly shaped. As they are solid nacre they tend to have a very high lustre.