Check out this article, it has information about all aspects of setting up natural decor. Down the list and then further down is infor for types of stones and how to test them.
One thing is where they came from.
Some rocks around yards and buildings are often sprayed with insect poisons, plant killers and other stuff. So it is good to know where they came from if you collected them for your tank and might need some good scrubbing with water and baking soda and vinegar.