Purple Coin Ball
From the Super Mario Wiki
The Purple Coin Ball is a ball, appearing in Super Mario Galaxy 2, with a Purple Coin inside of it. It is similar in appearance to a Star Ball and behaves in the same manner, in that Mario or Luigi can roll around on it by tilting the .
The Purple Coin Ball only appears in the second mission of the Rolling Coaster Galaxy; Purple Coins on the Rainbow Road. Mario or Luigi must maneuver the Purple Coin ball to the end of the course, collecting 100 Purple Coins as he goes. When he reaches the goal, the Purple Coin Ball will shatter, releasing an extra Purple Coin within it for Mario or Luigi to collect.