Pearls and cultured pearls

Color : Pink, silver, cream, golden, green, blue, black
Mohs’ hardness : 3-4
Specific gravity : 2.60-2.78   
Cleavage : none 

Fracture: Conchoidal or scaly
Crystal system : Microcrystalline
Chemical composition : 84 – 92% calcium carbonate, 4 – 13% organic substances, 3 – 4% water
Refractive index : 1.52-1.66

Refractive index (black): 1.53-1.69
Double refraction: weak or none

Pearls are produced by molluscs, rarely by snails. They consist of mother-of-pearl, which is mainly calcium carbonate (in the form of aragonite), and an organic horn substance (conchiolin) which are formed concentrically around microcrystals. Although the Mohs’ hardness is only 3 – 4, pearls are extraordinarily compact and it is difficult to crush them. 

The size can vary between a pin head and a pigeon’s egg. The largest pearl ever found weighs 450 carats (1800 grains); it is in the South Kensington Geological Museum in London. 

The typical pearly luster also called “orient” is produced by the overlapping platelets of aragonite and skins of conchiolin nearer to the pearl surface. This formation also causes the refraction of light and the resulting colors of the spectrum which can be observed on the pearl surface. The color of the pearl varies with the type of mollusc and the water, and is dependent on the color of the upper conchiolin layer. If the conchiolin is irregular distributed, the pearl becomes spotty. 

Care can help to preserve pearls. Dryness, as well as humidity, are detrimental. Pearls are very sensitive to acid, perspiration, cosmetics and hair spray.

Cultured pearls are not imitation, but a natural product which has been produced with man’s help. Today cultured pearls amount to 90% of the total pearl trade.

The principle behind pearl culturing is simple. Man causes the mollusc to produce a pearl by insertion of a foreign body. In China as early as the 13th century, small leaden figures of Buddha were fixed to the inner wall of the mollusc shell so that they would be covered with pearl material. Round pearls were first produced by Swedish naturalist, Carl V. Linne, in 1761. In 1893 the Japanese, Kokichi Mikimoto, managed to produce semi-spherical pearls.

The most important element in the production of a pearl is the tissue, not the foreign body. Theoretically one can do without  the bead, the “working time”  of the mollusc is shortened. Only one layer of nacre is necessary for the bead to receive a typical pearl luster. 

The molluscs remain in the water for 3 to 4 years; by then the layers around the bead are of about 0.8 – 1.2mm. If they remain longer in the water, there is a danger that they will become ill, die or mar the shape of the pearl. No mother-of-pearl is secreted after the 7th year. Cultured pearls with a thin pearl covering are considered inferior.

A special variations of pearls is the black tahitian pearl.