The Complete Guide to Sauna Stones: Which to Use, Why They Matter, When to Replace Them

Most people only think about sauna stones once — when they buy the sauna. But the stones are the heart of the room. Without them, an electric heater is just a glowing coil, and a wood stove is just a fire in a box. The stones are what turn heat into steam.

If you have ever stepped into a Finnish sauna and felt that wave of soft, enveloping heat after water hits the rocks — that is löyly, the steam Finns describe with the same word they use for "spirit." It is produced not by the heater, but by the stones. They store the energy, hold it patiently, and release it as vapour the moment water touches them.

Choose the wrong stone, and the result is a sharp, wet blast that doesn’t spread. Choose the right one, and the steam fills the room evenly, hangs in the air, and lasts long enough that you reach for the ladle again before the first pour has fully faded. The stone matters more than most people realise.

What sauna stones actually do

A sauna stone is, technically, a thermal battery. It is heated by the element or fire, stores the energy in its mass, and releases that energy in two ways: continuously as radiant heat, and explosively as steam when water flashes off its surface. To do that job for years, the stone needs three properties: high density (so it stores a lot of heat per litre), high thermal shock resistance (so it doesn’t crack when cold water hits it at 400°C), and a chemical composition that doesn’t off-gas anything harmful.

That rules out an enormous number of rocks. Granite cracks. River stones — collected from beds and beaches — can be catastrophically dangerous, because trapped water inside them turns to steam and shatters them with the force of a small explosion. Decorative stones, marble, slate, and anything labelled "landscape" are not sauna stones, no matter how beautiful they look.

The two stones used in Finland

Olivine diabase — the workhorse

Olivine diabase (also called dolerite) is the standard sauna stone supplied with most Finnish heaters. It is a dark, fine-grained igneous rock formed when magma cools beneath the earth’s surface. Its density is around 3.0 to 3.5 g/cm³, which is high enough to hold serious thermal mass, and its mineral composition gives it excellent resistance to thermal shock. Industry sources describe its lifespan as roughly two to three years in regular home use.

If your iHKA sauna comes with stones included, this is most likely what you have. They are the right answer for the vast majority of home installations.

Peridotite — the upgrade

Peridotite is denser still — by some measurements roughly 15% denser than olivine diabase — which translates directly to more thermal mass per stone. The practical consequence is steam that lasts noticeably longer between pours, and a stone that resists fracture longer over its lifetime. Industry sources put the lifespan of premium peridotite at four to five years in regular home use.

Finnish sauna tradition has used peridotite for centuries. It is the connoisseur’s choice and an excellent investment if you sauna often.

Why size and arrangement matter as much as type

Sauna stones are typically sold in the 5–10 cm size range, and that’s not arbitrary. Smaller fragments pack too tightly, blocking airflow around the heating elements; larger stones leave hot spots and gaps in coverage. The sweet spot allows convection — air rising through the stone bed, what Finns sometimes call the "chimney effect" — which both protects the heating elements and produces an even heat across the room.

Equally important is how the stones are stacked. They should be loose, not packed. There must be gaps between them, ideally enough that you could see daylight through the heater if you held a torch behind it. A tightly packed stone bed traps heat against the elements, causes them to overheat and trip the safety, and — in extreme cases — burns out the heater entirely.

The annual stone check

Both Harvia and IKI publish the same core advice: rearrange your stones at least once a year, and more often with frequent use. The yearly check takes thirty minutes and protects the most expensive component in your sauna. We covered the full procedure in our maintenance guide, but the short version is:

• With the heater fully cold and powered off, lift each stone out one by one.
• Discard any that are cracked, flaking, or noticeably reduced in size.
• Sweep out fragments and dust from the base of the heater.
• Inspect the heating elements (electric) or fire pipe and mesh (wood) for any damage.
• Repile loosely, with small gaps between stones for airflow.
• Top up with new stones of the same type and size as needed.

“A weak löyly is almost never a heater problem. It is, nine times out of ten, a stone problem — and one that thirty minutes of attention will solve.”

When to replace, and when to top up

Stones don’t fail all at once. They wear gradually — small cracks, surface flaking, fragments at the base of the heater. The annual check is also your replacement opportunity. For typical private use, top up gradually as stones break down, and plan a full refresh every three to five years. For intensive or commercial-style use, a complete annual replacement is the standard recommendation.

Use replacement stones of the same type and size as your originals. Mixing types is technically possible but tends to produce uneven steam character. If you started with olivine diabase and want to upgrade, do the swap as a complete change rather than a partial one.

Water on the stones — the part most people get wrong

Finnish sauna culture has strong opinions about water. Use clean tap water — that part is uncontroversial. But never use sea water, salty water, or water with high mineral content. Salt accelerates corrosion on the heating elements and the mesh, and will void heater warranties. High-iron water leaves orange deposits on the stones and the heater body. If your local tap water is very hard, occasional rinsing of the stones during the annual check helps.

Sauna scents — eucalyptus, birch, pine — are fine if they are sold specifically for sauna use. Follow the dosage on the bottle, which is always a small amount diluted in water. Never pour essential oils or scented products neat onto the stones; they ignite, smoke, and stain.

The pour itself

One last detail worth knowing. With the Harvia Cilindro, you can throw water on the side of the column for a soft, gentle löyly, or straight on top for a sharp burst. With the Mini IKI, the same principle applies via the position of your pour. This is part of the craft of sauna bathing — adjusting the steam to the moment, the company, and the mood. It’s also why Finns are so particular about their stones. The right ones make every pour feel intentional.