The spheres range in size from a few centimetres to over 2 metres (6.6 ft) in diameter, and weigh up to 15 tons.[3] Most are sculpted from gabbro,[3] the coarse-grained equivalent of basalt. There are a dozen or so made from shell-rich limestone, and another dozen made from a sandstone. They appear to have been made by hammering natural boulders with other rocks, then polishing with sand.