They will use the calcium from the water to build their shells, so its the opposite of what your thinking.
I used to collect these from a lake behind my house when I was a kid.
They live in the sand and can be found at the surface of the sand or just below it.
I knew every lake in the area and only one lake had them.
I don't think pebbles would be good for them...
I don't know much about keeping them long term in captivity though.
I do have experience with saltwater clams though:
