I'm assuming granite is safe to use in an aquarium, but I know it can be dangerous to assume. any pros or cons to using granite (rocks, not gravel), in an aquarium?
Most granites are totally safe for the aquarium. I use granite pea gravel in a couple tanks and use and have used granite rocks with no problems.
The only thing I look for in granite is any "metallic" looking flakes in the rock, these rocks I reject. I don't know if they would pose a problem, if it looks metallic, I always worry about using it.
I agree with what Burtess has said. One test that you can do to test if a rock is safe for an aquarium or not is if you put a few drops of vinegar on the rock, if it fizzes it is not safe if it doesn't fizz than it is safe.