Chinese Hand Exercise Ball
When talking a walk, i usually carry these. It makes me looks like a Asian sage.
Don't buy those with decorative inlaid brass wire (called Cloisonné) on the ball surface. they look pretty, but if you dropped or chipped the ball, the metal wire may stick out.
Get pure marble stone, quartz, or solid steel ball. Also, most of them chime. I prefer them not, especially if you work in the office.
Also, get 5.5 cm (2.16 inches) diameter at least, for Asian sized male hand. (white men or black men's hands are bigger.)
you may find it difficult to roll on your hand at first, but after a while, it becomes easy. Smaller balls are useless.
Crystal ball, quartz ball, or gem stone balls, also work. You just have to find the right size.