A 12th century Bishop of Rennes asserted the value of thunderstones as a divinely appointed means of securing success in battle, safety on the sea, security against thunder, and immunity from unpleasant dreams

During the Middle Ages many of these well-wrought thunderstones were venerated as weapons, which during the “war in heaven” had been used in driving forth Satan and his hosts. Hence, in the 11th century the Byzantine emperor sent to the Holy Roman emperor a “heaven axe”; and in the 12th century, a Bishop of Rennes asserted the value of thunderstones as a divinely appointed means of securing success in battle, safety on the sea, security against thunder, and immunity from unpleasant dreams.