Nearly every kettlebell on earth is gray cast iron, and the recipe matters. Iron becomes castable when you dissolve carbon into it, around three percent by weight, along with a dose of silicon, and as the melt cools that carbon precipitates out as microscopic flakes of graphite laced through the metal. The flakes turn a fracture surface gray, which named the alloy, and they change how the material behaves. Graphite damps vibration, which is why engine blocks are gray iron and why a kettlebell lands with a thud where a steel plate would ring. Those flakes also let the molten metal flow thin and fill fine detail, at the price of brittleness. Gray iron does its best work as a compact lump, and a compact lump with a handle is the whole job description.