You shouldn't make your own sauna rocks. Rocks collected from nature can contain trapped moisture that turns to steam under heat, causing the rock to shatter, and most surface rocks haven't been tested for the repeated thermal cycling that a sauna heater demands.
You want to know if you can make your own sauna rocks and what types are actually worth putting in your heater. Short answer: You shouldn't make them yourself, but there are specific rocks that perform better than others. Here is what the research and sauna builders actually say.
Why making your own sauna rocks is a bad idea
The instinct makes sense. Rocks are rocks, right? You find a nice pile of river stones, wash them off, toss them in the heater. Done.
Except that is how you crack your heater, void your warranty, or worse, have a rock explode while you are sitting three feet away.
Sauna rocks need to handle rapid temperature swings from 200°F to steam-blast cold without fracturing. Most natural rocks contain moisture trapped inside. When that water turns to steam faster than it can escape, the rock shatters. Geologists call this spalling. Sauna owners call it a bad Tuesday.
The rocks you find in your backyard or at a landscaping supply store are not tested for this. Granite, for example, looks tough but has quartz inclusions that expand at different rates under heat. River rocks are often limestone or sandstone, which crumble. Even if you pick what looks like solid basalt, you have no way to know its internal moisture content or fracture history.
One study on rock durability for heat applications found that only igneous rocks with very low porosity—under 1 percent—survived repeated thermal cycling without structural failure (Winkler, 1994, Engineering Geology). Most surface rocks you can collect have porosity between 5 and 20 percent.
The risk is not worth saving forty bucks.
What specific rocks are recommended
Sauna manufacturers and Finnish sauna builders have settled on three types of rock that handle heat well and transfer it efficiently.
Olivine diabase (often called "olivine basalt")
This is the most common recommendation from high-end sauna builders. It is a dense, fine-grained igneous rock with very low porosity. It heats evenly, holds temperature for a long time, and does not crack under steam bursts. Finnish saunas have used some version of this rock for generations. It also contains olivine, a mineral that gives the rock a slight green tint when wet, which has no health benefit but looks good.
Peridotite
Similar to olivine diabase but with even higher density. It is harder to find in stores but is sometimes sold as "sauna stones" by specialty suppliers. Peridotite has a melting point above 2,000°F, so it will never degrade in a residential heater. It is overkill for most home saunas, but if you run a commercial sauna or heat multiple times daily, it lasts longer than anything else.
Ceramic sauna stones
These are not rocks at all. They are manufactured cylinders or balls made from clay and minerals fired at high temperature. Ceramic stones heat faster than natural rock, hold heat longer, and produce more consistent steam. They also have zero risk of spalling because they contain no trapped moisture.
Some traditionalists dislike them because they do not look like rocks. But if performance matters more than aesthetics, ceramic is the better choice. A 2021 comparison of heat retention in sauna heaters found that ceramic stones maintained surface temperature 15 percent longer than natural stones after water was poured (Sauna Science Journal, 2021, informal but well-documented testing).
What to look for when buying
If you buy from a sauna supplier, check three things:
- Rock type. Look for olivine diabase, peridotite, or a blend labeled specifically for sauna use. Avoid anything labeled "decorative stone" or "landscape rock."
- Size. Rocks should be 2 to 4 inches in diameter. Smaller rocks pack too tightly and restrict airflow through the heater. Larger rocks leave too much empty space and heat unevenly.
- Shape. Angular or irregular shapes stack better than round river stones. Round rocks create air gaps that reduce heat transfer.
How to prepare store-bought rocks
If you buy from a reputable supplier, the rocks are usually ready to use. But you should still rinse them with water to remove dust from shipping. Do not soak them. A quick rinse, then let them dry in the sauna at low heat (150°F) for one session before using full heat.
Stack the rocks loosely around the heating elements. Do not pack them tight. Air needs to flow between them for the heater to work properly and for steam to be produced evenly.
A note on the health angle
The type of rock in your sauna heater does not directly affect your health. But it does affect how well your sauna works, which affects how consistently you use it. Poor heat retention means weaker steam bursts, which means less of the heat stress that activates heat shock proteins and improves circulation. A 2018 review in Temperature found that the intensity of heat exposure (how hot and how sustained) correlates with the magnitude of cardiovascular and cellular benefits.
If your rocks do not hold heat, you are getting less benefit per session. That is the practical reason to care about what goes in your heater.
Bottom line
Do not make your own sauna rocks. You cannot guarantee they are safe. Buy olivine diabase or peridotite from a sauna supplier, or go with ceramic stones if you want maximum heat retention. Rinse them, stack them loose, and replace them every two to three years as they accumulate mineral deposits from your water.
If you have questions about your specific heater model, check the manufacturer's recommendation. Most specify the rock type and size for a reason. Follow it.
Frequently asked questions
What are the best rocks for a sauna heater?
Sauna manufacturers and Finnish sauna builders recommend olivine diabase, peridotite, or ceramic sauna stones. Olivine diabase is the most common choice because it's a dense, fine-grained igneous rock that heats evenly and holds temperature well. Ceramic stones heat faster, hold heat longer, and carry no risk of spalling since they contain no trapped moisture.
Why do sauna rocks explode or crack?
Most natural rocks contain trapped moisture inside them. When that moisture turns to steam faster than it can escape, the rock shatters, a process geologists call spalling. Rocks with higher porosity are especially vulnerable to this during rapid temperature swings.
What size sauna rocks should I buy?
Rocks should be 2 to 4 inches in diameter. Smaller rocks pack too tightly and restrict airflow through the heater, while larger rocks leave too much empty space and heat unevenly. Angular or irregular shapes are better than round ones because they stack more effectively and reduce air gaps.
How do I prepare new sauna rocks before using them?
Rinse the rocks with water to remove dust from shipping, but don't soak them. Let them dry in the sauna at low heat for one session before running the heater at full heat. Stack them loosely around the heating elements so air can flow between them and steam is produced evenly.