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.

South Sea pearls are the largest cultured ... The Pinctada maxima oyster can take up to five years to produce a single South Sea pearl, whereas more common freshwater oysters can take as little ...

Pearls are made by marine oysters and freshwater mussels as a natural defence against an irritant such as a parasite entering their shell or damage to their fragile body. The oyster or mussel slowly ...

Oysters are ... valuable. Pearls commonly found in jewellery are typically round and highly valued, especially if they are natural pearls. Both saltwater and freshwater pearls are assessed based ...