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Pearl Gemstone Information

Pearl is one of the few natural gemstones, albeit organic, that needs little or no processing to prepare it for use in jewellery. The surface of pearl has a natural lustre that does not need cutting or polishing; unlike most other gem materials. This allowed pearls to be used for adornment in ancient times, when craft technology was as an early stage.

From those ancient times until the beginning of the 1900's only natural pearls were available and they were so rare that only the very rich could own them. Since the inception of the cultured pearl industry, pearls have become both affordable and widely available. Various shapes and sizes appeared on the market and totally round pearls, of almost calibrated sizes, became available for the first time.

Basic cultured pearl manufacturing process

Cultured pearls are either fresh water pearls produced in rivers or saltwater pearls produced in saltwater lagoons or protected coastal areas.

Both types of pearl are produced by inserting a seed material (nucleus) into a mollusc - a freshwater mussel or saltwater oyster. The molluscs are then suspended by apparatus below the water in the river or the sea for a long period of time. During which, the immune system of the mollusc coats the foreign invader (the seed) with layers of nacre (calcium carbonate in the form of aragonite, cemented by conchiolin).

A number of seeds are implanted into a single mollusc to increase the yield. Numerous  materials of various shapes are used for the nucleus, for example, a perfectly round sphere of mother of pearl may be used, to produce a perfectly round pearl. The colour of the pearl is the same as that of the nacre that lines the mollusc shell, typically white, pink, silver and creamy gold colours. Natural black pearls also occur, but are very rare, Tahiti is well known for its cultured black pearls.

The quality and price of a cultured pearl increases with the amount of layers that coat the nucleus. This is determined by the amount of time allowed for the pearl to develop. A larger nucleus would enable a small number of nacre layers, a shorter time to produce, but a poorer quality product. A larger number of nacre layers is more desirable.

Natural pearls, where the seed is typically a parasite or a small grain of sand, are comprised of 100% nacre, minus the size of the fortuitous invader.

Physical Properties

Stone type:
Pearl is an Organically produced gemstone.
Crystal System:
Microcrystalline.
Chemical Composition:
Calcium carbonate.
Colour:
Natural body colours include: white, pink, yellow, green, brown, grey and black. The pearly lustre itself can be responsible for displaying tints of colour that are different from the body colour.
Lustre:
Pearly lustre describes the pearls surface light effect.
Pleochroism:
Not present.
Dispersion:
None.
Hardness:
2½ to 3½ on the Mohs' scale of hardness.
Toughness:
Poor: Pearls need protection from acids, including secretions from skin, perfumes and deoderants. Higher temperatures may dry out pearls. They aslo need protection from physical blows.
Cleavage:
None.
Density in gm/cc:
2.60 to 2.85
Double refraction:
Yes.
Refractive index:
1,52 to 1.56

Common Treatments

Cultured pearls are dyed to a wide range of colours.

The top layer of nacre may be removed to improve the colour or remove damage and imperfections from older pearls. In the past this was the job of a person called a pearl doctor.


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