Just a footnote:
Until the 1930s, there was a thriving clam digging industry on the upper Mississippi. My wife was given a ring by her Great Aunt containing two "river pearls". One is pink and the other is a beautiful light blue. While scarce, these fresh water pearls were not ultra unusual.
Since these mollusks are no longer harvested, the pearls have become rare finds.
Wow. Learned something there. I didn't even know about 'river pearls'. TY.