Cool I've got sprit of salt will give that a go.Put some strong vinegar or acid on them and see if they fizz. I would be concerned that they are limestone
Thanks for the adviceI believe nitrate bottle 1 or ammonia bottle 2 from the API test kits can be used for an acid test.
Also, I'm sure I'll be corrected if wrong, most rocks will be fine, worst case they will increase you hardness over time but a weekly waterchange will reset this.
Although you should take into consideration the requirements of your livestock. Not all will handle changing parameters.