Found this on the net, searched just to make sure:
There is absolutely no reason not to use granite in a freshwater aquarium. It is typically an inert stone, and will not affect water quality. If you collect it yourself however, make sure you sterilize it by soaking the pieces in hot water, then rinsing thoroughly and allowing to cool before use.
I got mine from a landscape yard, rinsed it with the hose a few times and good to go.