I have a nice short book on aquascaping I found cheap on Amazon it's been out for awhile it's called "Creating a Natural Aquarium". It's a good beginners book.
They mention to be sure a rock is safe for an aquarium and won't raise PH or change water hardness to put a few drops of vinegar on the rock if it fizzes it's fine if not it's alkaline and could change your parameters. I have rocks I've collected from creeks around my area that are sitting on the side of my house. Many have fossils imbedded in them.
Have you tried this test? How else can I determine if rocks are ok to use in my aquarium? FYI I've always gardened organically so no chemicals should be present on them. My neighbors on the side of the house they have been sitting don't spray either.