Freshwater cultured pearls come from freshwater mussels and are produced by Japan, China, and The United States. The most famous type of freshwater cultured pearl is the Biwa (pronounced bee-wah) pearl which use to come from mussels grown in Lake Biwa, Japan's largest freshwater lake. Because of pollution, production has stopped. Freshwater shell and pearl mussels are from the family Unionidae, from which about 20 different species are commercially harvested. The tones of the freshwater pearls are dictated by the mother shell. White is the most common, followed by pink. Other colors depend on the type of mussels. Big washboard mussels (Megalonaias nervosa) usually have pink pearls, as do the wartyback mussel (Quadrula nodulata). Threeridge mussels (Amblema plicata) have pearls in shades of blue-green and lavendar. Muckets (Actinonaias ligamentina) produce fineThat is why saltwater pearls demand a higher price in the market. Less than 5 in 10 will survive the nucleation process. Of the survivors, about 40% will successfully encircle the shell nucleus irritant with nacre. Overall, less than 5% of pearl output can be considered "high quality." The most desirable shape for saltwater pearls is round. The most popular colors for saltwater pearls are white, cream, golden and black. Saltwater pearl market occupies 75% of all pearl market today. Saltwater pearls are graded on a scale from AAA (Excellent) - Commercial (Poor) grades. Below is a saltwater grading chart with explanations.

AAA - Perfectly Round; Excellent Luster; Clean Nacre of 95% Inclusion Free Surface; Excellent Matching; Top 1%.
AA+ - Visibly Round; Excellent Luster; Clean Nacre of 90-95% Inclusion Free Surface; Excellent Matching; Top 5%.
AA - Off Round; Good Luster, Good Nacre of 80-90% Inclusion Free Surface; Good Matching; Top 15%.
A/A+ - Off Round to Baroque ; Moderate Luster; Medium Nacre of 70% Inclusion Free Surface; Moderate Matching.
A - Very Off Round to Baroque ; Moderate/Low Luster; Moderate to Heavily Blemished Nacre; Moderate Matching.
Commercial - Asymmetrical Baroque; Moderate/Low Luster; Heavily Blemished Nacre; Heavy Inclusions; Poor Matching.