Where’s the clam?
This person found this weird glass object sitting at the bottom of a tidal pool. If you don’t know, a tidal pool, or a tide pool, are pockets of water that are left over when the ocean recedes. They can be filled with all sorts of creatures, which is why the OP claims that there might’ve been a clam inside of this glass object.
As for what the glass object was, it was created by the hands of man. Someone might have melted a piece of glass in a bonfire, which got washed away by the ocean before settling in a tidal pool.