From the Facebook Page of the Maryland Department of Natural Resources.

 

 

The Maryland Department of Natural Resources says that a couple of 6th and 7th graders were enjoying the outdoors at Winters Run in Harford County when they found a pearl in a freshwater clam (Corbicula)! Corbicula is a non-native clam in Maryland waters, but like all clams, it has the capacity to produce pearls. This is obviously very rare, so these kids got lucky!

Wikipedia says that the Corbicula fluminea is a species of freshwater clam native to eastern Asia which has become a successful invasive species throughout North America, South America, and Europe. Corbicula fluminea is commonly known in the west as the Asian clam or Asiatic clam.