from the google
"Glass is actually "harder" than iron on the Mohs scale, and spark plug ceramic (technically called "aluminium oxide ceramic") is much harder than glass. Aluminium oxide ceramic actually rates a 9 on the Mohs scale; diamonds are 10, glass is 6.5, and iron is 4.5. That's the key to the whole thing, and why it's surprisingly hard to break a window with a hammer and surprisingly easy to break it with a small, light little shard of innocent white spark plug ceramic."