Devices and methods are disclosed for the centrifugal casting of spheres, particularly solid and hollow spheres having a gradient internal density. Molten or self-hardening casting material is introduced into a hollow spherical mold and the mold made to rotate about two axes intersecting at the center of the mold. The resulting centrifugal forces cause the casting material to be evenly distributed about the inner surface of the mold. If a mixture of casting materials of different densities is used, the densest material will settle closer to the walls of the mold, while less dense materials will move closer to the center. The casting material is hardened and the resulting sphere is removed from the mold. One or more valves may be employed to transport fluent materials into and out of the mold. If gas is removed from a partially-filled mold prior to casting, the product will be a hollow sphere having an interior pressure less than ambient. Spheres made according to the present invention have a wide range of uses including insulation and bearing balls.