Natural freshwater pearls occur in mussels for the same reason that saltwater pearls occur in oysters. Foreign material, usually a sharp object or parasite, enters a mussel and cannot be expelled. To reduce irritation, the mollusk coats the intruder with the same secretion it uses for shell-building, nacre.

To culture freshwater mussels, workers slightly open their shells, cut small slits into the mantle tissue inside both shells, and insert small pieces of live mantle tissue from another mussel into those slits. In freshwater mussels that insertion alone is sufficient to start nacre production.

Most cultured freshwater pearls are composed entirely of nacre, just like their natural freshwater and natural saltwater counterparts.

For more information on freshwater pearls, visit NOVA at the following link:

http://www.pbs.org/wgbh/nova/pearl/freshwater.html