If you want to try to boil water using heated stones and are unsure about which stones to use then B&Q may hold the answer. A recent acquisition of both Japanese and Scottish Black decorative stoves I have found work well. I heated them up to red heat and plunged into cold water. The only ones to break were already faulted as could be seen as cold dark lines against the glowing red. I assume these rocks are Basalt but am open to other suggestions.