Most kettlebells are made of a single, solid piece of cast iron. This offers an extremely solid construction that can withstand heavy use. Other kettlebells feature a softer exterior covering, such as vinyl or a rubberized material, that is designed to minimize damage to your floors if the kettlebell is dropped. Kettlebells can also be made of a heavy material, such as cement, with an exterior covering. While not as rugged as solid-metal kettlebells, they are often more economical.