Lava Rocks vs Sauna Stones: Are They Interchangeable?

What lava rocks are

Lava rocks, often sold for gas grills, fire pits, or landscaping, are pieces of volcanic rock, usually basalt or a related vesicular rock. They are lightweight, porous, and full of air pockets left by escaping gas as the lava cooled. That porosity makes them excellent for heat dispersal in a grill, but it also means they absorb moisture easily.

In a sauna, you will occasionally see lava rocks offered as an alternative to traditional sauna stones. Some manufacturers of sauna heaters, however, do not recommend them. The reason comes down to how the stone behaves under repeated thermal stress and exposure to water. For more about the different rock types, see our guide to sauna stone types.

What makes a stone suitable for a sauna heater

A good sauna stone is dense, heavy, and resistant to thermal shock. It should be able to sit at high temperatures for hours, then receive a ladle of cool water, without cracking or spalling. Common choices include peridotite, diabase, olivine, and soapstone. These stones have a high heat capacity, meaning they store a large amount of heat and release it slowly as gentle steam.

The Finnleo FAQ notes that pouring water on heated rocks increases humidity, and it recommends sprinkling one ladle of water on the rocks for a wet sauna experience. It also warns that using too much water is indicated by water leaking out the bottom of the heater. That advice assumes the stones are doing their job: absorbing heat and converting water into steam. A porous lava rock may absorb the water it is hot, then release it unevenly, or it may crack from the sudden temperature change.

Dense stones also create a more comfortable loyly (the steam that rises from the stones). The heat stored deep in the stone evaporates the water quickly, producing a soft, enveloping steam rather than a short burst of weak vapor. Learn how to properly load your heater stones to get the best performance.

How lava rocks differ from sauna stones

The main differences are density, water absorption, and thermal stability.

Density: Lava rocks are light and airy because of their vesicles. Sauna stones are much heavier for their size. Dense stones store more heat per volume, which means your heater can maintain a steady temperature and produce good steam over time.

Water absorption: The pores in lava rock soak up water. When you throw water on the stones, a porous rock holds the water instead of instantly turning it to steam. That water then cools the surface of the stone, which can cause the outer layer to flake off, a process called spalling. Over time, the rock can crumble into small pieces that block airflow through the heater and reduce its efficiency.

Thermal stability: Lava rock is formed when molten lava cools quickly, leaving a glassy, brittle structure. Repeated heating and cooling, especially with the shock of cold water, can cause it to fracture. Sauna stones are chosen for their ability to withstand these cycles. For more on selecting the right size and type, see our guide on stone size and heater type.

Risks of using lava rocks in your heater

Using lava rocks in place of sauna stones can be a safety concern. If a rock cracks or spalls, it can produce sharp fragments that may fly when you pour water on the heater. More importantly, small pieces can fall into the heater element area and accumulate, reducing airflow and potentially causing the heater to overheat.

Some lava rocks also contain minerals that may release an odor when heated. While most volcanic rock is inert, you do not want to introduce a stone of unknown origin into your hot room, especially if it was sold for landscaping, where it may have been treated or contaminated.

For these reasons, the safest choice is to use stones that are specifically sold as sauna stones, from a sauna supplier or a heater manufacturer. Check your heater manual for the recommended stone type and size. If you already have stones that are cracking or crumbling, it is time to replace them.

Choosing the right stones for your heater

When you pick stones for your heater, look for stones that are dense, smooth, and free of cracks. The common sizes range from about 2 to 4 inches in diameter, but follow the recommendation in your heater manual. Larger stones allow more airflow between them, which helps the heater reach temperature quickly, while smaller stones create more surface area for steam production.

You can learn more about the different rock types in our guide to sauna stone types, and about how to load them in our guide to loading heater stones. If you are setting up a new heater, also see our guide on stone size and heater type.

If you already have stones and are wondering whether they are safe to use, look for signs of cracking, crumbling, or excessive dust. When stones start to break down, it is time to replace them, as described in our guide on when to replace sauna stones.

Frequently asked questions

Here are answers to common questions about lava rocks versus sauna stones.

• Can I use lava rocks from a grill store in my sauna? It is not recommended. They are unlikely to withstand the thermal stress of a sauna heater, and they may crack or spall.
• Are lava rocks and sauna stones the same material? They can both be volcanic in origin, but sauna stones are specifically selected for density, hardness, and thermal resistance. Lava rocks sold for grills are lightweight and porous.
• What happens if I use lava rocks anyway? They may absorb water instead of producing steam, crack under high heat, and create debris that can block airflow in the heater.
• How can I tell if a stone is a real sauna stone? Real sauna stones are dense, heavy, and smooth. If you tap two stones together, they should clink rather than thud. When in doubt, buy stones from a sauna supplier or the heater manufacturer.

What to pick for your use

Questions

Can I use lava rocks from a grill store in my sauna?

No, it is not recommended. Lava rocks sold for grills are lightweight and porous, and they may crack or crumble when exposed to the high, repeated heat of a sauna heater.

Are lava rocks and sauna stones the same material?

They can both be volcanic in origin, but sauna stones are specifically selected for density, hardness, and thermal resistance. Lava rocks for grills are chosen for heat dispersal, not for thermal storage.

What happens if I use lava rocks in my sauna heater?

Lava rocks may absorb water instead of producing a good steam burst, and they are prone to cracking or spalling. Over time, broken pieces can block airflow in the heater, reducing its efficiency and potentially causing overheating.

How can I tell if a stone is a proper sauna stone?

Proper sauna stones are dense and heavy for their size, with a smooth surface and few visible cracks. If you tap two stones together, a dense stone will produce a clear ringing sound, while a porous lava rock will sound dull.

How often should I replace sauna stones?

Stones should be inspected every few months. Replace them when you see significant cracking, crumbling, or when they no longer produce a good steam burst. Dust and debris from broken stones should be removed regularly to keep airflow through the heater clear.