Freshwater pearls are produced commercially by Hyriopsis cumingi (triangle shell) and Hyriopsis schlegeli (Biwa shell) in China, and other bivalve mollusks that live in lakes, riverbeds and creek bottoms in Japan (Biwa pearls and Lake Kasumigaura pearls), as well as the United States (Mississippi River Basin). In addition to the traditional white body colour, these pearls come in a rainbow of natural colours as brilliant as lavender, tangerine, mauve, aqua-silver, peach and every shade in between. Their unique shapes range from very baroque to nearly round and are almost indistinguishable in their shape from Akoya pearls.
The varied shapes include potato-shaped and stick pearls, rice-shaped and button pearls, coin-shaped and drop pearls, off-round and round pearls. A perfectly round freshwater pearl is extremely rare. Their sizes range from tiny seed pearls measuring 1mm or 2mm in diameter to 15mm and larger. Over the last decade or so, Chinese pearl farmers have greatly improved processes for growing cultured Freshwater pearls that are made up completely of nacre. They have also developed ways to reshape pearls by repositioning them during the growth process to result in more perfectly rounded ones. At these farms, each mussel is surgically implanted with 24 to 32 tiny pieces of mantle tissue on either side of the shell, a process known as nucleation (new-klee-AY-shun). Once they have been nucleated, the mussels protect their flesh from irritants by secreting nacre.
Freshwater pearls are nucleated in a different fashion to their counterparts. In lieu of the mother-of-pearl bead, the pearl farmers nucleate their mussels with only a piece of mantle tissue. This mantle tissue is not placed in the reproductive organ of the mussel but in the fleshy mantle tissue. While Freshwater pearls are typically tissue-nucleated, meaning they are composed entirely of nacre, the fragment of mantle tissue disintegrates as the mollusk coats the tissue resulting in a pearl made of solid nacre.