A cultured pearl is a pearl created by a oyster farmer under controlled conditions. Cultured pearls can be farmed using two very different groups of bivalve mollusk: the freshwater river mussels, and the saltwater pearl oysters. The cultured pearls on the market today can be divided into two categories. The first category covers the beaded cultured pearls, including Akoya, South Sea and Tahiti. These pearls are gonad grown, and usually one pearl is grown at a time. This limits the number of pearls at a harvest period. The pearls are usually harvested after one year for akoya, 2–4 years for Tahitian and South Sea, and 2–7 years for freshwater.