Only the strongest rock is able to survive constant heating and cooling and even the best ones will after time crack and crumble. They will no longer retain heat and will disintegrate, clogging the kiuas.
The best rocks for a sauna are those that have not been weathered, for example certain types of quarried rock. It is not recommended to use glassy rocks with a high quartz or iron content. Iron conducts heat very quickly and when in contact with water the heat gets trapped in a vapor shell and forms beads not steam. When this slides off the rock a very weak loyly is the result. Never use any rock that releases poisonous gas or horrible odors when you make your own sauna.
A quarried Finnish rock called peridotite is probably the perfect choice for your sauna. Kiuas makers ship this rock across the world but certain North American rocks work quite well too. Quarried basalt, black and fine-grained from the Sierra and Cascade ranges are ideal as well as hornblende which is found in many areas. Hornblende is a textured rock that has been re-crystalized at high temperatures so it is a good choice for sauna rock.
Use a geological surveying map from any Bureau of Mines or through the Government Printing office in Washington, D.C. to locate sources of these rock types.
When you make your own sauna learn about rock testing. This is important to avoid the danger of exploding rocks. To guarantee their safety, thoroughly heat a sample for two hours or more, then place it into a bucket of cold water and examine it for cracks. Once the rock has cooled down, hit it with a hammer or against another rock.
If it cracks or makes a soft grinding sound when rubbed against another rock, discard it and choose another source. If the rock survives the test you have a safe sauna rock. For a small fee a more elaborate test can be conducted by your local metallurgical laboratory.
How many rocks will you need and how big should they be? An ideal sauna rock is the size of a large potato. If the heat is replenished adequately, the amount of rocks can be as little as 55-75 pounds. If using a heat storage kiuas, which are only heated once, you need at least 264 pounds of rock. A savusauna takes anywhere from 600-1100 pounds of rock depending on the room size.
