Ok, I have a very ugly piece of live rock getting overgrown with mushrooms, and instead of cutting the shrooms, I just want to cut the rock they are on, what would be the best way to go about this, I've been using a dremel for now but its slow, I don't have a diamond/carbide bit, it would probably help if I got one. Would I be able to score it with the dremel and then chisel it and get accurate breaks.

Yes, that would work. I usually just use a hammer and chisel. Surprisingly, it almost always comes out the way I want.

i say a good wack with a hammer is good enough for me.

Hammer and chisel work very well, just fine those imaginer "sections" in the rock to split off. If you have fragging pliars (stainless steel cutting players) you can hack away at it slowly.

OK, I scored lines into the rock with a dremmel, and after doing that I took a pair of tin snips and just snipped pieces of the rock off. It worked really well.