Why are sauna heater rocks important? Sauna rocks store heat from the sauna heater. When the bather throws water onto the hot rocks, the steam will blend with the dry air in the room, giving the bather an enjoyable and relaxing atmosphere. Throwing water on the rocks is an essential part of sauna. That action and its effect are called löyly (an ancient word that means soul). We import our rocks from Finland where the select superior rocks of the correct type (Peridotite or Igneous) for heat storing capacity and long-lasting use in sauna heaters.