A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk, usually a conch. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes of pearls (baroque pearls) occur. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries, and because of this, the word pearl has become a metaphor for something very rare, fine, admirable, and valuable.
Pearls are valued as follows:
Nacre is the smooth surface that gives the pearl its beauty. When an irritant enters the oyster, it responds by coating the irritant with layers of nacre. The thicker and smoother the nacre the more valuable the pearl.
This is the intense glow that comes from within the pearl, this is produced by light entering the pearl and reflecting back through the layers of nacre. The quality of the luster is directly related to how even and smooth the layers of nacre are. A pearl with no reflection or a dull color is of poor quality.
With pearls, the cleaner the surface the more valuable it will be. A good quality pearl will have few blemishes or marks - of course a pearl is from mother nature and few minor blemishes should be expected on all but the finest strands.
A perfectly round pearl is the most sought after and the most rare of cultured pearls. So rare are round pearls that only 5-10% of a pearl farm's harvest will be even and round.
Colour does not greatly affect the price of pearls as it is mostly a matter of taste. When choosing a colour, a guideline is skin tone. Many women with fair or light coloured skin feel rose(pink) overtones look best while women with dark, olive or tan skin find black or cream overtones look best.
Pearls can range in size from 1mm seed pearls to huge 20mm South Sea pearls. Cultured pearls of 6 - 7.5mm are the most common, above this size the price jumps upward rapidly.