Hardcore markers suck, the paint in it isn't that hard to buff, you just need to scrub harder. and unless there was stealth ink inside the marker, i doubt thats the reason why it wont come out. get some paint thinner or alcohol/nail polish. it'll come right off. and snap, lava rocks work pretty decently but are hard to get a hold of depending on where you are. sand paper is more effective than rocks though, and it won't come off unless the windows have those plastic seals on them, but even then, if you were to use a rock and sandpaper on the plastic sealed windows, the sandpaper has a better chance of going through the seals, and making a mark on the window itself. the only upside to lava rocks are that they give you a better grip, you can do handstyles with them, and it looks better.