An external object, such as a sand grain or tiny parasite, comes into a mussel and cannot be excreted. To relieve irritation, the mollusk coats the gatecrasher with a secretion it uses for building it's shell, called nacre. In cultured freshwater pearl production, workers ease the mussel half shells apart and cut small slits into the mantle tissue inside both shells. Then they introduce minute pieces of live tissue from a different mussel into the slits. That is enough to start nacre production. The majority of cultured freshwater pearls are made entirely of nacre, just like their natural counterparts.
Culture of fresh water pearls began in Japan. After their successes with culturing saltwater pearls, Japanese pearl farmers experimented with freshwater mussels in Lake Biwa. The early freshwater commercial crops came out in the 1930s. The all-nacre Biwa pearls featured colors never seen in saltwater pearls. Depth of luminescence and lustre matched naturals. In fact, until recently, all fresh water pearls were called
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