Freshwater pearls are produced by Hyriopsis cumingi (triangle shell) and Hyriopsis schlegeli (Biwa shell) commercially in China, and other bivalve mussels that live in lakes, riverbeds and creek bottoms in Japan (Biwa pearls and Lake Kasumigaura pearls), as well the United States (Mississippi River Basin). Although most freshwater pearl information lists several areas of the world as home to pearl-producing mussels, the global freshwater market is overwhelmingly dominated by Chinese pearl farms, which account for nearly all freshwater pearls sold today. At freshwater pearl farms, each mussel is surgically implanted with 24 to 32 tiny pieces of mantle tissue, a process known as nucleation. Once the tissue has been inserted, a sac forms and cells begin secreting nacre (pronounced NAY-ker), forming a calcium-carbonate compound - a pearl. Over the course of 2 to 7 years, the mussels deposit layer upon layer of nacre around the growing gems, generally producing more than two dozen pearls clustered on the inside of each shell. After harvest, millions of pearls are sorted carefully and matched for size, shape, color, and quality. Once the sorting is complete, loose pearls are available for sale individually and strands of matched pearls are sold on temporary strings known as "hanks."