Purple Coin Ball
From the Super Mario Wiki
The Purple Coin Ball is a ball with a Purple Coin inside of it in Super Mario Galaxy 2. 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 Wii Remote.
It appears only in the second mission of the Rolling Coaster Galaxy; Purple Coins on the Rainbow Road. Mario or Luigi must manuever 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, revealing the Purple Coin within it to Mario or Luigi. If Mario or Luigi does not have 100 Purple Coins by this point, he will lose a life and must restart the mission, or if he has no more to lose, will receive a Game Over.