Pearls are produced in both salt and fresh water, but the difference is astounding. Usually, the salt water variety of pearls are of a better quality and are also more expensive than pearls found in salt water; however, the choice between fresh water and salt water pearls is completely personal.
For starters, pearls are made from shellfish. When an irritant, such as a piece of sand, invades the delicate lining of the creature, it secretes a smooth substance to coat the irritant, thus creating a pearl. Salt water and fresh water pearls are both made in the same fashion. This substance, known as nacre or mother of pearl, slowly builds up the irritant until a pearl is formed.