Leather, synthetic leather, and rubber are the most common medicine ball covers. Leather or synthetic materials are mostly used on slam balls, but some exercisers like the feel of a leather ball for bouncing as well. Rubber is by far the most common material. On some medicine balls, the rubber is smooth and “sticky,” on others, it’s heavily textured and nubby, and still others fall somewhere in between. Your personal preference is most important here, but remember that you’ll likely sweat quite a bit during your workout, so you need a medicine ball that’s easy to hold on to even when your hands are wet.