Large Decorative Rock

I have used what I believe to be red granite with no ill effects. If you are really paranoid you can do a few things to test to see if your rock will affect your water chemistry. The first is to fill a bucket with water and test for whatever parameters you are concerned about (pH comes immediately to mind, I have never tested for anything else when testing rocks). Add the rock, check back in a week or so and run your tests again to see if water params are different. You can also use bottle one of the Aquarium Pharmaceuticals Nitrate Test in order to see if the acid in the test bottle reacts with your rock. If you notice any fizzing your rock is likely to alter the pH of your water.

99% of the time, rocks won't change your water chemistry. There are some out there that will but few and far between. Some will leach a little itty bitty bit, but doing water changes like you should, it won't be enough to effect anything significantly.