if you see any orange in the rock... don't use it. there could be iron in it and that's really bad for your tank!

boiling is risky, although i've done it many times... you can also bake them at 300 degrees for 20 minutes... works great.

all in all... if it's a normal river rock... go ahead and use it. just don't use gardening rocks (lava rock) as they'll mess with your pH in a really bad way!