Small shards or pieces of ceramic spark plug insulator (the white part). When thrown at glass, they break it very quietly. They are used mostly for car burglaries in which the burglar wants to remain unheard.
Dan hit the driver's window with ninja rocks, then Joe ran up on the guy hit him in the nose after opening the door.
a ninja rock is a rock that sneaks up on you while walking or running and causes you to either twist, sprain, or break your ankle. At the very least it will make you say ouch that hurt. Most commonly happens at night when you cant really see whats on the ground so well.