I'm not sure what kind off pearl it is. I order mussels and as I was eating them I bit something hard and out popped a small pearl. I was a very happy person. Is it lucky to find one?
The more I hear about people finding natural pearls, the more I want to start ordering mussels and oysters, but I really don't eat out that often and I'm not even sure they are for sale here locally. I love hearing these stories.
Most mussels live in freshwater, but there are "Blue Mussels" that live in the Pacific Ocean, that are naturally saltwater mussels. (I have some nice blue pearls from these blue mussels that I need to get on my site.)
Most likely, your mussel was freshwater, and if so, then you have a natural freshwater pearl! Lucky you!
I hope you find something special to do with it. It will be a wonderful keepsake.