the following appears to be a coral, or sponge? It is not my own find, a friend gifted them to me, he said his fisherman grandpa, found them in the water near the island. They are heavy-ish, very hard and sound like rocks when hit them with each other and other rocks.
size: when placed next to each other 10cm x 4cm(to3cm) and 2cm thick.
Also appart from the obvious shell which looks modern, there is a tiny white pointy edge - snail -like shell with vertical lines all around, and a hole on its side, that could also be fossil, unless that it is supposed to be so white when baby. (photo1 and 2, left piece, top right circle, u can see the baby shells opening and hole)