Pearls are formed naturally within the mantle (soft tissue) of
shelled molluscs. These animals have shells made of calcium carbonate
(CaCO3), which is the same material from which pearls are formed. Most
pearls today come from oysters, mussels or clams but almost all shelled molluscs are capable of producing them.
A natural (wild) pearl is created without human intervention over the period of several years. If an irritant such as a parasite enters the shell of a mollusc a pearl sac is created around the intruder to protect the animal. Usually the foreign object enters while the shell valves are open for breathing or feeding.
Layers of calcium carbonate are held together by a compound called conchiolin, forming nacre, which is the same substance as the lining of their shells. Over time many layers are built one upon the other forming the complete pearl. The inner layers are typically brown and the outer layers a yellowish/white colour.
The translucent layers of the pearl reflect, refract and diffract light which gives pearls their beautiful lustre. Pearls with more, thinner layers, will be more lustrous than pearls with less, thicker layers. Light falling on the surface of the pearl is disturbed by the overlapping of the layers below and is seen as iridescence (the change in colour as the view angle or light source changes).
Cultured pearls are created by implanting a small piece of mantle tissue from a ‘donor’ shell into the recipient shell. This causes the formation of a pearl sac where calcium carbonate will be created and a pearl is born. Cultured pearls can be created in either fresh water or salt water shells, and a spherical bead may or may not be inserted. Most cultured saltwater pearls are grown with a bead.
The shape of a cultured (beaded) pearl will be largely determined by
the shape of the bead which the mollusc builds layers over. A pearl
grown this way may be harvested in as little as 6 months.
From the outside it is not possible to tell whether a non-beaded pearl is natural or cultured. X-ray equipment is required to examine the centre of the pearl. A natural pearl will have concentric growth rings from the centre to the outside and a (non-beaded) cultured pearl will display a complex cavity at its heart. Beaded pearls have an obvious solid core surrounded by layers of nacre.
Imitation pearls can be made from
coral, conch shells or mother of pearl (the shell lining) and can look
quite real. A microscope can reveal an imitation pearl from a natural or
cultured pearl. Imitation pearls have a completely smooth surface
whereas natural and cultured have a slightly rough surface due to the
layers of nacre. Rubbing two pearls together (or on your teeth) can
reveal a slight grittiness if they are natural or cultured which won’t
be felt with imitation pearls.
The value of a fine quality wild (natural) pearl can be very high, a matching set of stranded pearls can run into hundreds of thousands of dollars. The New York Cartier store on 5th Avenue (which was a mansion at the time) was acquired in 1917 by Pierre Cartier in exchange for a double strand of matched natural pearls he had been collecting over many years. The value was estimated to be around $1,000,000 at that time.
The factors that determine the value of natural pearls include size, shape, lustre, colour and the quality of the surface. The ideal is a perfectly round and smooth shape, and shapes other than round are called baroque. The advent of cultured pearls saw a large drop in price and many women in the 1950’s were able to afford a pearl necklace, the natural pearls becoming an exclusive niche in the jewellery industry.
Black pearls are usually not black, but darker shades of silver, blue, grey, green, purple or peacock. Peacock is a mixture of several shades, similar to a peacocks tail. Cultured black pearls such as Tahitian pearls are valued because of their rarity. In common with most ocean or salt water pearls, they can only be implanted with one with one pearl at a time, unlike freshwater mussels which can have multiple simultaneous implants.
Prior to culturing, black pearls were very rare as few white pearl oysters naturally produced black pearls and black pearl oysters often did not produce pearls at all.
Pearl farming of cultured pearls began in 1916 by Mitsubishi but after some signs of success, the project was interrupted by the death of Tatsuhei Mise’s death in 1931 and again by the start of the 2nd world war. In the early 1950’s projects were started up in Australia and Burma with the involvement of Japanese technicians from Mitsubishi’s pre-war efforts.
Pearls are found in a range of eight basic shapes. Perfectly round are the most valuable and the rarest. Semi-round pearls can be disguised in jewellery by cleverly mounting them to look perfectly round. Drop and pear shapes are often used in earrings and pendants and may be referred to as teardrop pearls. Baroque pearls are irregular in shape, circled pearls have rings or ridges, buttoned pearls are a slightly flattened round and oval is a slightly elongated round.
White and black are the most popular colours, but small numbers of purple, green, blue, pink, champagne are also found in natural pearls.