PearlsAfter pieces of eight and gold doubloons, pearls are one of the treasures most often associated with pirates - a recent example being, of course, the movie Pirates of the Caribbean: The Curse of the Black Pearl. This connection is possibly because many natural pearls come from the sea - and all are associated with water.
What Are Pearls?Unlike many gemstones, pearls are organically formed. A pearl is created when a living creature - usually a mollusc such as an Akoya oyster - reacts to an irritating foreign body such as a sharp piece of grit or particle of food. If the mollusc can't expel the intruder, it attempts to "neutralise" it.
To smooth off the irritant, the mollusc coats it with layers of the same material that it uses to build its own shell - aragonite and conchiolin, a combination known as nacre (mother of pearl). If these layers build up properly then they can form a pearl, with the multi-layered structure giving the gem its distinctive shimmering lustre.
Pearls are usually "pearly white" however can come in a variety of colours resulting fom the organic pigments in the conchiolin. Pearls you can buy in the stores have sometimes been dyed to produce different colours. Apart from the creamy white, the colour most often associated with pearls is probably black, in particular the Tahitian black pearl.
Pearls are described as either saltwater or freshwater depending on their origin. The former have grown in the sea, the latter in fresh water such as inland lakes.
Natural vs Cultured PearlsTruly natural pearls are those that have occured spontaneously in "wild" molluscs without any human intervention. These are extremely rare and valuable. Earrings, a necklace or other such jewelry made from natural pearls would have been a great find for a swashbuckler of old!
Most real pearls you buy today are cultured - in effect cultivated "farmed". A small "core" is inserted inside the mollusc in order to stimulate pearl production. To the untrained eye these look the same as a natural pearl, however laboratory analysis will reveal the difference. Whereas natural pearls are almost all nacre around a tiny irregular core, cultured pearls usually consist of a much thinner layer of nacre around a larger spherical core.
Of course even cultured pearls are to an extent natural, so they're sometimes described as "cultured natural".
The first successfully created cultivated pearls were created by Kokichi Mikimoto of Japan. Mikimoto Pearls remain one of the most well-known brands on the market today.
It's said that some people can tell the difference between the two types of pearl by gently rubbing them along a tooth. Somehow I can't see grizzled old sea dogs bothering with such subtleties!