Rounded Olivine Diabase Sauna Stones, 3–5 cm (15 kg box)

HUUM's CLIFF and STEEL heaters hold their stones in tight spaces around the elements, so HUUM calls for small 3–5 cm stones. These are rounded olivine diabase, a dense natural stone that stores heat well. HUUM prefers round stones in its heaters because water runs down between them and turns to steam gradually, which it says makes the steam milder. Rinse the stones before loading, stack them loosely so air can move, and replace any that crack or crumble. Stones don't come with the heater, so order the full load at the same time.