Pebble tools were made using equipment such as anvils and hammer stones. It also forms from the outflow of tropical cyclones or the anvils of cumulonimbus clouds. The outer octahedral cavity is pressed by 8 steel outer anvils. Modern anvils are generally made entirely from steel. These discarded shells may accumulate around the anvils in sizable middens, sometimes for generations. In the tropics, these thunderstorms occasionally produce copious amounts of cirrus from their anvils. The curved anvils are then removed. Some anvils, mainly European, are made with two horns, one square and one round. In the climax to the entr'acte this rhythm is hammered out on eighteen anvils. However tuned anvils are available as musical instruments, albeit unusual. European anvils are sometimes marked in kilograms. This effect is often studied using diamond anvils. It appears as several anvils clustered together. The pressure-transmitting medium is enclosed by a gasket and the two diamond anvils. Commonly pairs of anvils tuned a third apart are used.