It just came into my mind, that I have seen in Ireland some sort of black stone the same kind of stone they used to make blackboards of in old times. Children had one each insted of notebooks. The structure of the this black stone is that it kind of is formed in layers.

In Danmark we use the type of stone on the roofs, I do not know the name in English. In Danish it's "Skifer" Well this has not much to do with music, but I hope it is okay, even if I'm wrong. regards anne....