How Pearls are Formed
There are essentially three types of pearls: natural, cultured and imitation.
Natural Pearls form when an irritant - usually a parasite and not the proverbial grain of sand - works its way into an oyster, mussel, or clam. As a defense mechanism, a fluid is used to coat the irritant. Layer upon layer of this coating, called 'nacre', is deposited until a lustrous pearl is formed.
A cultured pearl undergoes the same process. The only difference is that the irritant is a surgically implanted bead or piece of shell called Mother of Pearl. These 'seeds' or 'nuclei' are most often formed from mussel shells. Quality cultured pearls require a sufficient amount of time - generally at least 3 years - for a thick layer of nacre to be deposited, resulting in a beautiful, gem-quality pearl. Lower-quality pearls have often been 'rushed' out of the oyster too quickly (sometimes a year or less) and have a too-thin coat of nacre.
Pearls can come from either salt or freshwater sources. Historically, saltwater pearls were rounder and had a better nacre than freshwater pearls, while freshwater pearls tended to be very irregular in shape, with a puffed rice appearance the most prevalent. However, improvements in freshwater pearl farming techniques have narrowed that gap, with freshwater pearls now exhibiting great roundness and deep luster.
The culturing process usually takes several years. Mussels must reach a mature age, which can take up to 3 years, and only then can be implanted or naturally receive an irritant. Once the irritant is in place, it can take up to another 3 years for the pearl to reach its full size and nacre thickness. Of the pearls produced, only approximately 5% are of sufficient true gem-quality for top jewelry makers, yet a pearl farmer can figure on spending over $100 for every oyster that is farmed, whether a gem-quality pearl is produced or not.
Imitation pearls are a different story altogether. In most cases, a glass bead is dipped into a solution made from fish scales. This coating is thin and may eventually wear off. One can usually tell an imitation by rubbing it across the teeth: Fake pearls glide across your teeth, while the layers of nacre on real pearls feel gritty. The Island of Mallorca is known for its imitation pearl industry, and the term "Mallorca Pearls" or "Majorica Pearls" is frequently (though inaccurately) used to describe these pearl simulants.