Beach pebbles/rocks

[hillegom]

As said you do not want the minerals in the rock to leach out so pick rocks carefully.
Rocks will explode if heated too fast in the air ( as when you throw them in a fire.)
Thats because the water in the rock turns to steam and cannot escape fast enough.
Boiling rock in water is ok because the temp will never rise higher than 100 deg C
So no steam as long as the rocks are submerged