I'v done the river rocks, but I always boiled them to kill any bacteria on them. Just let it cool down before you put it in your tank, though this could be a ok way to keep your tank warm, as rocks hold heat rather well.
Definetly don't want any limestone in a tank, it will mess with your pH. That's what the vinegar test is for! :)
Edit: Just googled it, and apparently marble, limestone, and other "calcareous" (not sure what that means XD) rocks will fail the vinegar test. All calcareous rocks will mess with your pH too! Who would of thought marble would react with water? I love google <3