Lol yeah i should of mentioned that, it was pretty sharp on most edges so i quickly rubbed the slate together to 'fade' them out abit!I did smash a big 'rock' of slate up a few months ago for my tank and it has been fine
The only thing you have to watch for when you are smashing your own slate is to be careful of the edges which can be razor sharp and dangerous for you and your fish!
I'd suggest wearing eye protection maybe a hard hat (dont want a sharp bit stuck into ur head, slate was flying everywhere when i done it..
I did go all out with a massive sledge hammer though
Slate & wood imo make perfect for tanks