I think it's normal to be honest. I had a few larger pieces that I used a chisel on to break apart and when I did the inside of the broken pieces were brown like that. Nothing to worry about. You won't even see it once the algae sets over it.
The only one I had which is about a 1" chisel I bought from Home Depot. One word of caution... do it outside. You will have to smack the hell out of it with a hammer and it makes a heck of a mess. Bits of rock will fly too.