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Sauna stones aren’t decorative. They are functional thermal components that determine loyly quality, heater lifespan, and session consistency. The difference between the right stone and the wrong stone is the difference between soft, enveloping steam and a wet blast of poorly converted water vapor. In the worst case, the wrong stone can crack explosively and send fragments across the sauna room.
This guide covers the major stone types used in electric and wood-burning sauna heaters, with specific thermal properties, expected lifespans, and recommendations by heater type and usage pattern.
Why Does Sauna Stone Choice Matter?
Sauna stone selection directly determines loyly steam quality, heater element lifespan, and session consistency. The wrong stone can crack explosively, damage heating elements, and produce harsh, low-quality steam instead of the soft, enveloping vapor that defines a good sauna experience.
Heat Capacity and Loyly
When you throw water on sauna stones, the water absorbs thermal energy from the stone surface and converts to steam. The quality of this steam depends on two factors:
Stone surface temperature: Above 300C, water flash-vaporizes on contact, producing fine, dry steam (good loyly). Below 250C, water vaporizes incompletely, producing coarser, wetter steam that feels heavy and harsh.
Stone thermal mass: The total thermal energy stored in the stone mass determines how many water throws the stones can handle before their surface temperature drops below the quality threshold. More mass at higher temperature equals more sustained loyly.
Stone material directly affects both factors. Dense stones with high specific heat capacity store more energy per kilogram and maintain surface temperature longer after water contact.
Thermal Shock Resistance
Every water throw subjects stones to extreme thermal shock: a 350-400C surface contacted by 10-15C water. This creates rapid, uneven thermal contraction that generates internal stress. Stones with poor thermal shock resistance crack, split, or in extreme cases, fracture explosively.
Material crystalline structure and mineral composition determine thermal shock resistance. Some stones handle thousands of cycles. Others fail within months.
Heater Element Protection
Stones sit directly on or around heating elements. Stone fragments from cracking can damage element insulation, cause short circuits, or block airflow. Choosing stones with low fracture rates protects your heater investment.
What Are the Best Types of Sauna Stones?
Olivine diabase is the best all-around sauna stone for most installations, while peridotite is the premium upgrade for superior loyly quality and longer lifespan. Granite and river rocks should never be used due to poor thermal shock resistance and explosive fracture risk.
Olivine Diabase
Olivine diabase is the standard sauna stone worldwide and the most commonly supplied stone with Finnish heaters. It is a dark, dense igneous rock composed primarily of pyroxene, olivine, and plagioclase feldspar.
Properties:
| Property | Value |
|---|---|
| Density | 2.9-3.1 g/cm3 |
| Specific heat capacity | 0.82-0.88 kJ/(kg-K) |
| Thermal conductivity | 2.0-2.5 W/(m-K) |
| Thermal shock resistance | Good |
| Color | Dark grey to black |
| Typical size range | 5-15 cm |
| Expected lifespan | 2-3 years (heavy use), 4-5 years (moderate use) |
| Price (20kg box) | $30-50 |
Performance analysis: Olivine diabase has adequate density and specific heat capacity for good loyly production. At 3.0 g/cm3 and 0.85 kJ/(kg-K), a 50kg load of olivine diabase at 380C stores approximately 16,150kJ of thermal energy above room temperature. Each 200ml water throw absorbs approximately 500kJ, meaning the stone mass can theoretically handle 32 throws before reaching room temperature. In practice, surface temperature drops faster than core temperature, so quality loyly is sustained for approximately 5-8 throws in a 50kg load before surface temperature drops below 300C.
Thermal shock resistance is good but not exceptional. After 200-400 heating cycles, micro-fractures accumulate and stones begin to crack. Cracked stones should be replaced because they have reduced thermal contact area and can produce fragments.
Verdict: Olivine diabase is the correct default choice for most sauna installations. It offers the best balance of performance, availability, and cost. If a heater manufacturer includes stones, they are almost certainly olivine diabase.
Peridotite
Peridotite is a dense, ultramafic igneous rock composed primarily of olivine (60%+ by volume) with pyroxene. It is the premium sauna stone, prized for superior density and heat retention.
Properties:
| Property | Value |
|---|---|
| Density | 3.2-3.4 g/cm3 |
| Specific heat capacity | 0.84-0.90 kJ/(kg-K) |
| Thermal conductivity | 2.8-3.5 W/(m-K) |
| Thermal shock resistance | Very good |
| Color | Dark green to black |
| Typical size range | 5-15 cm |
| Expected lifespan | 4-5 years (heavy use), 6-8 years (moderate use) |
| Price (20kg box) | $60-90 |
Performance analysis: Peridotite’s 15% higher density compared to olivine diabase translates directly to 15% more thermal mass per unit volume. For the same physical space in your heater, peridotite stores more energy.
The higher thermal conductivity (2.8-3.5 vs 2.0-2.5 W/(m-K)) means peridotite heats more evenly from core to surface and recovers surface temperature faster after a water throw. This produces more consistent loyly quality across consecutive throws.
Thermal shock resistance is meaningfully better than olivine diabase. Peridotite’s olivine-dominant composition creates a more uniform crystalline structure with fewer weak planes for fracture propagation. Practical result: peridotite stones last roughly twice as long as olivine diabase before needing replacement.
A 50kg load of peridotite at 380C stores approximately 17,100kJ (versus 16,150kJ for olivine diabase). The 6% increase in stored energy is modest on paper but compounds with the faster surface temperature recovery to produce noticeably better loyly in back-to-back throw sequences.
Verdict: Peridotite is the upgrade choice for users who prioritize loyly quality and want to minimize stone replacement frequency. The 50-80% price premium over olivine diabase is justified by the performance improvement and the doubled lifespan.
Vulcanite (Volcanic Rock)
Vulcanite encompasses several volcanic rocks including basalt and various lava stones used in sauna applications. Quality varies widely depending on specific source and composition.
Properties:
| Property | Value |
|---|---|
| Density | 2.6-3.0 g/cm3 |
| Specific heat capacity | 0.80-0.85 kJ/(kg-K) |
| Thermal conductivity | 1.5-2.2 W/(m-K) |
| Thermal shock resistance | Moderate to good |
| Color | Dark grey, black, sometimes reddish |
| Typical size range | 5-12 cm |
| Expected lifespan | 2-3 years (heavy use), 3-5 years (moderate use) |
| Price (20kg box) | $25-45 |
Performance analysis: Vulcanite performs below olivine diabase on most metrics. Lower density means less thermal mass. Lower thermal conductivity means slower heat distribution and slower surface recovery after water throws. Thermal shock resistance is acceptable but inconsistent, depending on the specific volcanic source.
Some volcanic stones contain gas pockets (vesicles) that can trap moisture. When these pockets heat rapidly, the trapped water converts to steam and can cause the stone to fracture or pop. Always inspect volcanic stones for visible pore structure and avoid stones with large visible vesicles.
Verdict: Vulcanite is an acceptable budget alternative to olivine diabase but offers no advantages. If the price difference is minimal (often the case since both are in the $25-50/box range), choose olivine diabase.
Granite
Granite is widely available and commonly suggested by well-meaning but poorly informed sources. It is one of the worst choices for sauna heater stones.
Properties:
| Property | Value |
|---|---|
| Density | 2.6-2.8 g/cm3 |
| Specific heat capacity | 0.79-0.84 kJ/(kg-K) |
| Thermal conductivity | 2.1-3.5 W/(m-K) |
| Thermal shock resistance | Poor |
| Color | Varied (grey, pink, black, speckled) |
| Typical size range | Variable |
| Expected lifespan | 6-18 months (often less) |
| Price | Cheap (often free) |
Performance analysis: Granite’s thermal properties are superficially adequate. The density and specific heat are in the workable range. The problem is thermal shock resistance.
Granite has a heterogeneous mineral composition: quartz, feldspar, and mica in varying proportions. These minerals have different thermal expansion coefficients. When heated rapidly, they expand at different rates, creating internal stress at grain boundaries. When cold water hits a hot granite surface, the rapid, uneven contraction causes cracking along these boundaries.
The result: granite stones in a sauna heater crack within months. Cracked stones produce fragments that can damage heating elements, reduce airflow, and in some cases, fracture explosively (though this is more common with river rocks than quarried granite).
Verdict: Don’t use granite in a sauna heater. The material is fundamentally unsuited to the thermal cycling environment. Despite being free and readily available, the rapid degradation and element damage risk make it a false economy.
River Rocks / Field Stones
This is the “worst case” category and must be addressed because it is a common DIY mistake.
River rocks and field stones of unknown composition should never be used in sauna heaters for two reasons:
- Unknown thermal shock resistance: Without knowing the mineral composition, you can’t predict fracture behavior.
- Moisture inclusion risk: River rocks may contain sealed moisture pockets from their time submerged. When heated to 300-400C, this trapped moisture creates steam pressure inside the stone. The stone can explode, sending fragments at high velocity across the sauna room. This is a genuine safety hazard.
Verdict: Never use river rocks or unidentified field stones. The risk of explosive fracture is real and documented.
How Do Sauna Stone Types Compare in Performance?
Peridotite ranks highest for heat retention and longevity, olivine diabase offers the best value, vulcanite is an acceptable alternative, and granite and river rocks should be avoided entirely.
| Stone Type | Density (g/cm3) | Specific Heat (kJ/kg-K) | Thermal Shock | Lifespan (heavy use) | Price (20kg) | Recommendation |
|---|---|---|---|---|---|---|
| Peridotite | 3.2-3.4 | 0.84-0.90 | Very good | 4-5 years | $60-90 | Best performance |
| Olivine Diabase | 2.9-3.1 | 0.82-0.88 | Good | 2-3 years | $30-50 | Best value (default) |
| Vulcanite | 2.6-3.0 | 0.80-0.85 | Moderate-good | 2-3 years | $25-45 | Acceptable alternative |
| Granite | 2.6-2.8 | 0.79-0.84 | Poor | 6-18 months | Cheap | Don’t use |
| River rocks | Unknown | Unknown | Unknown | Unknown | Free | Never use |
How Does Stone Mass Affect Loyly Quality?
More stone mass means more stored thermal energy and more sustained loyly. Doubling the stone mass roughly doubles the number of quality steam throws before the surface temperature drops below 300C. A 100kg stone load can sustain 10-13 throws versus just 2-3 for a 20kg load.
The total stored thermal energy in your stone mass is the product of three variables:
Stored energy (kJ) = Mass (kg) x Specific heat (kJ/kg-K) x Temperature rise (K)
For a target stone temperature of 380C (653K) and a starting temperature of 20C (293K), the temperature rise is 360K.
| Stone Mass | Olivine Diabase Energy (kJ) | Peridotite Energy (kJ) | 200ml Throws Before 300C Surface |
|---|---|---|---|
| 20kg | 6,120 | 6,480 | 2-3 |
| 30kg | 9,180 | 9,720 | 3-4 |
| 50kg | 15,300 | 16,200 | 5-7 |
| 80kg | 24,480 | 25,920 | 8-10 |
| 100kg | 30,600 | 32,400 | 10-13 |
The “throws before 300C surface” column is an approximation because surface temperature drops faster than core temperature. But the trend is clear: doubling stone mass roughly doubles loyly endurance.
This is why the Harvia Forte (100kg) outperforms the Finlandia FLB-80 (30kg) on loyly quality by such a wide margin. It isn’t a subtle difference. It is a 3x-4x advantage in sustained steam capacity.
How Should You Stack Sauna Stones in a Heater?
Stack the largest stones (10-15cm) loosely around the heating elements at the bottom, medium stones (7-10cm) in the middle, and the smallest stones (5-8cm) on the top throwing surface. Always maintain at least 10mm clearance between stones and elements, with visible air gaps throughout.
Proper stone stacking affects heater performance, element lifespan, and loyly quality. The principles apply to all heater types but are most critical for enclosed wall-mount units where airflow through the stone mass is constrained.
Layer 1: Around the Elements (Bottom)
- Use the largest stones (10-15cm diameter)
- Arrange them loosely around the heating elements
- Maintain at least 10mm clearance between stones and elements
- The goal is to protect elements from direct water contact while allowing air to circulate freely
Critical: Don’t pack stones tightly against elements. Stones in direct contact with elements create hot spots that accelerate element degradation and can cause localized element burnout. The air gap is essential for both airflow and element cooling.
Layer 2: Middle Fill
- Use medium stones (7-10cm diameter)
- Fill the space above the elements
- Maintain gaps between stones for airflow (you should be able to see light between stones when looking from above)
- Don’t stack in ordered rows. Use a natural, slightly random arrangement that creates irregular air channels
Layer 3: Top Surface (Throwing Surface)
- Use smaller stones (5-8cm diameter)
- Create a relatively flat top surface for water throwing
- Ensure stones are stable and won’t shift when water hits them
- Smaller stones on top create more surface area for water contact, improving steam conversion efficiency
Common Stacking Mistakes
Too tight: Tightly packed stones restrict airflow, causing uneven heating and poor convection. The bottom stones overheat while the top stones remain cool. Loyly quality suffers because only the top layer is at throwing temperature.
Too loose: Excessively loose stacking wastes volume and creates large air pockets that reduce total thermal mass. Some gaps are necessary, but excessive gaps are wasteful.
Stones too small at the bottom: Small stones near elements can shift during thermal cycling and make contact with elements. Always use the largest stones at the bottom to create a stable protective layer.
Mixing stone types: Different stone types expand at different rates when heated. Mixing olivine diabase with granite or volcanic stones can cause stones to crack against each other as they expand unevenly.
How Often Should You Replace Sauna Stones?
Replace olivine diabase every 2-3 years with heavy use and every 3-4 years with moderate use. Peridotite lasts roughly twice as long at 4-5 years with heavy use. Inspect monthly for cracking, powdering, and fragments, and remove any visibly degraded stones immediately.
Stone replacement is a maintenance task that many sauna owners neglect. Degraded stones have reduced thermal performance and increased fracture risk.
When to Replace
Replace your stones when you observe any of the following:
- Visible cracking: Any stone with cracks longer than 20% of the stone’s diameter should be removed.
- Powdering/crumbling: Stones that produce powder or granular debris when handled have lost structural integrity.
- Reduced loyly quality: If loyly feels heavier and wetter than when stones were new, degraded stones may be the cause.
- Reduced heat-up speed: Cracked and degraded stones have lower effective thermal mass and poorer thermal conductivity, which can increase heat-up time.
- Stone fragments in the heater: Fragments below the stone layer can block airflow and damage elements.
Recommended Replacement Schedule
| Stone Type | Heavy Use (5+ sessions/week) | Moderate Use (2-4 sessions/week) | Light Use (1 session/week or less) |
|---|---|---|---|
| Olivine Diabase | Every 2-3 years | Every 3-4 years | Every 5-6 years |
| Peridotite | Every 4-5 years | Every 5-7 years | Every 8-10 years |
| Vulcanite | Every 2-3 years | Every 3-4 years | Every 4-6 years |
Partial Replacement
You don’t always need to replace all stones at once. During regular inspections (monthly is recommended), remove and discard any cracked or degraded stones and replace them with new ones of the same type. This extends the life of the overall stone load and maintains consistent performance.
When the majority of stones (more than 50%) show signs of wear, do a full replacement. Mix old and new stones of different degradation levels creates uneven thermal behavior.
Where Should You Buy Sauna Stones?
Purchase sauna stones from heater manufacturers (Harvia, HUUM, Finlandia, Sawo) for pre-sorted olivine diabase, or from specialty sauna suppliers for premium peridotite. Avoid hardware store decorative rocks, landscaping stone, and river rocks, as these have uncontrolled mineral content and potentially dangerous moisture inclusions.
Manufacturer-Supplied Stones
The easiest option. Harvia, HUUM, Finlandia, Sawo, and other manufacturers sell pre-sorted olivine diabase stones in 20kg boxes. These are graded for size consistency and inspected for obvious defects.
Specialty Stone Suppliers
For peridotite and other premium stones, specialty sauna supply companies offer sorted and graded stones. Narvi (Finland) and several online sauna retailers carry peridotite in appropriate sizes.
What to Avoid
- Hardware store “decorative rocks” (unknown composition, possible coatings)
- Landscaping stone (uncontrolled mineral content, may include limestone which degrades rapidly)
- River rocks from any source (moisture inclusion risk)
- Ceramic stones/balls (functional but produce different loyly character than natural stone. Some users find the steam harsh)
What Stones Should You Use for Each Sauna Heater Model?
Match your stone type to your heater’s capacity: use olivine diabase as the default for all heaters, and consider peridotite for heaters with 55kg or less stone capacity where maximizing each kilogram of thermal mass matters most.
| Heater Model | Stone Capacity | Recommended Stone Type | Stone Cost (full load) |
|---|---|---|---|
| HUUM Drop 9kW | 55kg | Olivine diabase or peridotite | $83-248 |
| Harvia Cilindro 9kW | 80kg | Olivine diabase or peridotite | $120-360 |
| Harvia Forte 9kW | 100kg | Olivine diabase (peridotite cost-prohibitive at this mass) | $150-225 |
| Finlandia FLB-80 | 30kg | Olivine diabase or peridotite | $45-135 |
For the Harvia Forte’s 100kg capacity, a full peridotite load would cost $300-450, which is significant. Most Forte owners use olivine diabase for the lower two-thirds and peridotite for the top throwing layer, combining cost management with optimal surface performance.
What Is the Best Sauna Stone to Buy?
Use olivine diabase as your default sauna stone for the best balance of thermal performance, availability, and cost. Upgrade to peridotite if you want measurably better loyly and are willing to pay a 50-80% premium. Never use granite, river rocks, or stones of unknown composition.
Use olivine diabase as your default sauna stone. It offers the best combination of thermal performance, availability, and cost. Upgrade to peridotite if you want measurably better loyly quality and are willing to pay the 50-80% price premium. The doubled lifespan of peridotite partially offsets the higher purchase price over time. Never use granite, river rocks, or stones of unknown composition. Replace stones on schedule rather than waiting for visible failure. Proper stone selection and maintenance is one of the lowest-cost, highest-impact improvements you can make to your sauna experience.