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 appears only 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, revealing the Purple Coin within it to Mario or Luigi.