The disc is andesite most likely a common volcanic rock that is more mafic , darker minerals overall , than rhyolite which is the volcanic rock that is chemically identical to granite. Each class of rock has extrusive and intrusive analogs with the rhyolite extrusive. The intrusive analog to andesite would be diorite. Neither diorite or andesite are widely used in bead making which is a shame as andesite tends to have an attractive open grain such as in this piece.