It is intially a small yellow ball covered in small bumps, however when Nine bounces it, it expands, turns black and spikes explode from the bumps. Nine cannot control the spikes but can usually control the color.
This is first seen in the abandoned house in West Virginia when Nine and Four are investigating their chests. It explodes and Four has to roll out of the way to avoid being impaled. Nine later uses it to blast open a door leading into the Dulce govenment base in New Mexico.