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 and 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 one-hundred Purple Coins as he goes. When he reaches the goal, the Purple Coin Ball will shatter, releasing the Purple Coin within it for Mario or Luigi to collect.