The most popular and effective method for creating cultured pearls are made from the shells of freshwater river mussels harvested in the midwestern states of the U.S., from Canada to the Gulf of Mexico. Shells with the common names, "Washboard" "Maple Leaf" "Ebony" "Pimpleback" and "Three Ridge" are popular for use in pearl culture due to their compatibility with the host animal, and the nacre they are to be covered by. These high-quality and sought-after shells are first sliced into strips and then into cubes. The edges and corners are ground down until they are a roughly spherical and then milled to become perfectly round, and brought to a highly polished finish.