A rock sauna is a traditional Finnish sauna that uses sauna rocks in its operation. The sauna rocks are heated and held with the sauna heater. In order to create the vapor, the sauna bather tosses water onto the hot rocks to increase the humidity in the sauna room.

Rock sauna owners always use proper sauna rocks in their saunas. Very often sauna rocks are provided with the Finnish-style saunas. But sauna owners who enjoy frequent dips in their rock saunas will likely someday have to replace old sauna rocks with new ones.

At any rate, even the best sauna rocks eventually crack and crumble under the intense pressure of the sauna's fluctuating temperature and humidity levels. Disintegrating, sauna rocks start to produce less steam, and there is also a possibility of clogging your sauna heater. That's why, your task is to learn to differentiate good sauna rocks from bad sauna rocks.

There are rocks that can explode in the sauna or give off poisonous or offensive odors. Sauna owners should avoid using such rocks. But you shouldn't use rocks that contain sulphur minerals (minerals with a yellowish color and/or metallic luster) or asbestos minerals. Those rocks with sulphur minerals do not pose a health risk as they wear down very quickly.

There is a way to avoid placing potentially explosive rocks in your sauna heater. For this you have to thoroughly heat a sample for two hours or more. Then drop it into a pail of cold water and look for cracks. Test the rock again when it is cool. Hit it with a hammer or against another rock. You should remove that rock if it cracks or makes a soft grinding sound when rubbed against another rock. If it survives, you have a safe sauna rock.

Peridotite, a quarried Finnish rock, is one of the most popular and one of the most common rocks in the earth's upper mantle. This is a type of igneous rock, as are olivine and vulcanite which likewise perform well in saunas. Dark-colored igneous rocks are often used in saunas because these rocks contain magnesium and iron and therefore have a relatively high heat capacity. However, be sure to refer to the manuals and other literature that came with your sauna and sauna heater before you replace your sauna rocks.

You should use enough rocks to cover the heating elements of an electric sauna heater. Heating your sauna with a wood burning stove, the rocks should form a small mound that rises above the stove's rim. Don’t forget to pack the rocks loosely to allow for sufficient air circulation.