It's called a fitness ball, or a hand held fitness ball. There are marble, rusty jade, stainless steel. Jade feel better, stainless steel is too heavy not suitable for finger inflexibility people use. My personal advice is that the fingers are not flexible temporarily with large walnuts to exercise the fingers better, because the large walnuts themselves are lighter, suitable for the fingers are not flexible people in the hand rotation, not easy to fall. Moreover, the pitted surface of the walnut is good for massaging the acupoints on the palm of the hand. When the fingers are more flexible later to buy a jade, because jade can be nourished and feel good.
In fact, the earliest hand held in the hands of practicing fingers is the big walnut. Later came the jade fitness ball that the princes and kings liked to hold, and in modern times the marble and stainless steel fitness ball was derived.