The heart of any sauna is the heater. In fact, a sauna without a heater is simply a room made out of wood.
The heater has two basic functions - create heat and create steam. Stones that are piled on the heater retain the heat and also produce steam if you pour water over them.
Infrared heaters are an exception. They don't use stones, so you
Oil and gas heaters are way less popular than the other types of heaters.
One reason is because they’re relatively inefficient compared to the others.
Some people even complain about the odor associated with the burning fuel. But if they're properly vented, oil and gas heaters should have little or no discernible smell.
What about those sauna stones?
They need to have special qualities to be able to withstand the severe temperature changes.
Really, the stones can be any type of rock but some kinds are better suited for use in a sauna.
The stones shouldn't give off any odor when heated, and they should be able to withstand high heat without cracking.
Good sauna stones should not have any cracks, and it's better if they have a rough surface. That helps to release steam faster and retain heat better.
Popular sauna stones are peridotite and olivine, and both are available at sauna supply stores.