Types of Sauna Stones: Peridotite, Olivine, Diabase and More

What sauna stones actually do

A sauna heater is a box of rocks with a heating element hidden underneath. The rocks do two jobs: they absorb heat from the element and release it as soft radiant heat, and when you throw water on them they flash it into steam, the burst of heat the Finns call löyly. The type of rock changes both of those jobs in a real way.

The maker of traditional sauna heaters advises sprinkling one ladle of water onto the stones at a time. Dumping water is not recommended, and a clear sign that you have used too much is water leaking out the bottom of the heater. That guidance applies no matter whether your stones are peridotite, diabase or soapstone. For the full routine, see how to pour water on stones. Traditional saunas are typically heated between 150 and 195 degrees Fahrenheit with low humidity, according to Cleveland Clinic.

Peridotite: dense rock that stores heat

Peridotite is a dark, coarse igneous rock from the earth's mantle, packed with the green mineral olivine. Because it is dense, it stores a lot of heat in a small volume, which means it can keep producing steam after the heater cycles off, and it recovers quickly after you throw water.

Stones sold as peridotite often arrive as rounded, fist-sized pieces with a greenish-black look. They are a common factory fill in many electric heaters because they combine heat storage with durability, though like all stones they will crack and crumble over time. If you want to understand how peridotite compares head to head with diabase, read peridotite vs diabase stones, and check how many stones your heater holds before you order.

Olivine: the mineral behind the green

Olivine is a silicate mineral, not a rock on its own. When you see a stone labeled olivine, it is usually peridotite or another olivine-rich rock. The olivine gives the stone its olive-green color and contributes to its density and heat retention.

The practical difference between a peridotite stone and an olivine-labeled stone is mostly naming. What matters is the rock's density and how it behaves in your heater, not the precise mineral name on the bag. Stone shape and size matter as much as the mineral content, and stone size and heater type explains how to match them.

Diabase: fine grained and durable

Diabase, also called dolerite, is a dark igneous rock with a fine, even grain. It is dense and heavy, which lets it store heat well, and its fine grain resists cracking from thermal shock. That makes it a strong choice for heaters that run hot for long sessions.

Many electric heaters ship with diabase stones. If you are replacing stones in an existing heater, diabase is a safe, widely available option, though the best pick depends on how hard you run the heater and how often you throw water. Over time every stone wears down, so plan around when to replace sauna stones as part of your maintenance routine.

Soapstone, ceramic and lava options

Soapstone, also called steatite, is a soft metamorphic rock that holds heat for a long time. It gives a softer, more even heat, which some bathers prefer, but it is softer than igneous stones and can erode or shed dust with heavy water use. If you like to throw a lot of water, dense igneous stones are usually a better fit. Details are in soapstone in heaters.

Ceramic stones are a man-made alternative that heats fast and is uniform, but they do not store heat the way dense rock does. Lava rock is light and porous, heats quickly, and also cools quickly, so it suits low-temperature use more than a hot traditional sauna. River rocks are not a safe substitute: stones from a stream can trap water and crack apart in a heater, which is why the river rocks in heater warning exists.

Choosing and loading stones for your heater

The best stone for you is the one that matches how you use the heater. If you run a hot and fast sauna and throw water often, peridotite or diabase gives a strong steam response. If you prefer a milder heat and longer sessions, soapstone may feel better. Whichever you pick, the stones need to be sized to your heater. Most electric heaters specify a stone diameter range, commonly around 2 to 4 inches, and using stones that are too large or too small leaves gaps that hurt heat transfer.

Load the stones loosely so air can circulate and water can reach the heating element. Stacking them too tight blocks airflow and makes the heater work harder. The general guidance is to fill the basket without forcing, leaving some space between stones. After months of use, stones can settle and pack down, so it is worth restacking them periodically. The restack stones routine explains how often and how to do it.

When buying a heater, look for one that carries an ETL Listed Mark, which shows it has been independently tested to applicable safety standards. Intertek is an OSHA-recognized Nationally Recognized Testing Laboratory, so the mark provides the same safety verification as other certification marks.

When to replace sauna stones

All sauna stones eventually crack from repeated heating and cooling. A cracked stone still holds some heat, but a stone that has broken into pieces with sharp edges can damage the heater element or create hot spots. The general rule is to inspect your stones a few times a year and replace any that are broken, crumbling, or showing white dust. That white dust is a sign of erosion, and you can read more about white dust on stones.

If you are buying a new heater, the stones that come with it are usually chosen for that specific model. When it is time to replace them, you can mix stone types, but keep the size consistent and avoid filling the heater with pieces that are too small. The number of stones your heater holds affects both heat storage and steam output; see how many stones your heater holds to plan a refill.

What to pick for your use

Questions

What is the best type of sauna stone?

Dense igneous rocks like peridotite and diabase are the most common because they store heat well and handle repeated thermal stress. Soapstone stores heat longer but is softer, while ceramic and lava rock have different trade-offs. Pick based on your heater and how you use it, not on brand names alone.

Can I mix different types of sauna stones?

You can mix stone types, but keep the size consistent and avoid pieces that are too small. Mixing dense and light stones will change the heat store and steam production, so test a small batch first if you are unsure.

How often should I replace my sauna stones?

Inspect your stones a few times a year and replace any that are broken into pieces, crumbling, or covered in white dust. The exact timeline depends on how hard you heat the room and how much water you throw; a heavy user may need to replace sooner than a light user.

Are river rocks safe to use in a sauna heater?

No. River rocks can contain trapped water, which turns to steam and can cause the rock to explode inside the heater. Stick to stones that are sold for sauna use, such as peridotite, diabase, or soapstone.

Why do my sauna stones crack and make noise?

Cracking is caused by thermal shock when hot rock meets cold water. It is a normal part of a stone's life. A single stone may last several years, but eventually every stone will crack. If you hear loud cracking, you may be pouring water too aggressively.