Not a shell but from the water, pretty sure

[paul monfils]

Almost any bivalve mollusk (clams, oysters, mussels, scallops, etc.) can make pearls. It is how they neutralize small irritating bits of matter that get inside their shell, like sand grains, etc. They coat the irritating particle with the same material their shell is made of, and continue to coat it repeatedly so that it gradually becomes smooth and increases in size. The problem is, such pearls, being made of the shell material, are opaque, like the shell. Your little specimens appear to be somewhat transparent?