If you use the ceramic part of a spark plug to throw it into a glass window of a car, it will break. Even if it's just a small piece. The aluminum oxide of a ceramic or a porcelain has so much energy on it concentrated when it gets in contact with a glass breaking it. In this video, the guys tried it out. The first man threw the shard a bit too weak and the camera was not able to catch the other guy throwing it on the other window that's why it looked like the guy threw it on the right but the left glass broke.