Cav,
Those marble balls are a Chinese thing based on Feng Shui (a whole different topic). Regardless of the size of the ball, they fit perfectly inside the stand and "float" on a cushion of water about the thick of a human hair. It only takes 10 PSI to get them rolling, and even one like you are sitting next to that weighs probably a ton or more can be spun by hand.
You can buy them as small as for your desk. They are really cool I think.
Them Chinese.....what will they think of next.