Sauna rocks are not decorative. They store heat from the stove, release it slowly into the air, and when water hits them, they produce steam. The quality of your rocks directly affects the quality of your löyly.

What Makes a Good Sauna Rock

The ideal sauna rock is dense, non-porous, and thermally stable — meaning it can absorb and release heat repeatedly without cracking or spalling. It should also be free of minerals that release harmful gases when heated.

The most commonly recommended types are:

  • Olivine diabase — The gold standard. Dense, dark grey, and exceptionally durable. Resists thermal shock better than almost any other rock. This is what most Finnish sauna manufacturers use.
  • Peridotite — Similar properties to olivine diabase, widely available in Scandinavia.
  • Granite — Common and affordable, but quality varies widely. Avoid granite with visible mica flakes (the shiny bits), which can crack explosively under heat.
  • Basalt — Smooth, dense, and good heat retention. Works well in electric saunas.

What to Avoid

Never use rocks you've collected from a river, beach, or garden without knowing what they are. Sedimentary rocks like sandstone or limestone can fracture violently. Rocks with trapped moisture or air pockets are especially dangerous — they can explode when heated.

Also avoid any rock with a metallic or glassy appearance. These often contain minerals that oxidize or off-gas at sauna temperatures.

How Many Rocks Do You Need?

As a rule of thumb, you need roughly 40–60 kg of rocks for a residential sauna stove. More rocks mean more thermal mass — which means more consistent heat and better löyly. Under-fill the stove and the heat spikes and drops; over-fill and air can't circulate properly.

When to Replace Them

Rocks wear out. Over 1–3 years of regular use they become porous, brittle, and less efficient at holding heat. If your rocks have started to crumble, develop white patches, or no longer produce good steam, it's time to replace them. Wash new rocks and heat them slowly the first few times to cure them.