Some stones have properties that either make them dangerous or inefficient for holding heat from a fire. If you’re not familiar with which stones are dangerous to build a fire pit with, join me in an educational experience to learn about the 10 stones you should avoid using in a fire pit.
Rocks that contain water should not be used in a fire pit. This includes porous rocks that may contain water, such as sandstone, shale, pumice, and limestone. River rocks or rocks of all types that have been lying in a riverbed should not be used because they may contain water that will explode when they are heated in a fire pit.
Some types of rocks are prone to exploding in fire pits. The explosion sends rock chips and pieces of varying sizes with tremendous velocity.
These become projectiles that can cause severe injury. Some types of rocks should not be used because they are made of materials that do not heat, while others are too small to use.
Here, I explain the 10 types of rocks that should not be used, why you shouldn’t use them, and how to identify them.
Table of Contents
What Stones Should Not Be Used In A Fire Pit
1. Limestone
Limestone is a sedimentary rock composed of shell fragments and tiny fossils. It is comprised of calcite and dolomite which have been powerfully compressed over time.
It is a porous stone that is generally gray colored, or white, yellow or brown. It crumbles easily when exposed to acids and is prone to scratching.
It is commonly used for fireplace hearts, but it should not be used in fire pits. It is a porous rock that has many small holes throughout the structure.
Liquid can easily penetrate the exterior and cause the rock to explode when exposed to heating. Limestone is a soft stone that is also prone to scratching and discoloration. It is noted by its white chalky appearance
2. Sandstone
Sandstone is a sedimentary rock that is formed of tiny granules of sand that have been tightly compressed over time. It is a porous rock that holds water on the interior easily.
It is highly permeable, and one of the most likely rocks to contain water on its interior. It should never be used in a fire pit because of the high likelihood of explosion under heating.
It is a rock that is usually light and soft. It does not hold up well over time when used under extreme heating conditions.
Visually, it is identified by its porous texture, sedimentary layers, and it may be gray, white, blue, red, or a variety of different colors combined.
3. Pumice
Pumice is a porous igneous rock that was formed when hot volcanic lava is exposed to water or colder air. Bubbles form within the rock that weakens its overall structure and adds to its lightness and porous texture.
Pumice is a soft rock that is highly permeable to air and water. It is likely to contain pockets of water deep within the rock that can cause it to explode when exposed to extreme heat.
4. Shale
Shale is a common sedimentary rock that is found in over seventy percent of the crust of the earth. It is formed from compressed clay and mud.
It features a fine grain often in gray or black, but shale can occur in any color. It is usually found in oceans, rivers, and basins near sandstone and limestone deposits.
Most shale contains a high calcium content due to fossils included in its formation. Since shale is most often associated with bodies of water, the likelihood of it containing water within its interior is high.
This makes it combustible and unstable under conditions of extreme heating.
5. River Rocks
River Rocks are rocks that have formed in riverbeds. They are various rocks made of compounds that erode by water to form smooth round edges.
These rocks are more likely to contain water on the interior from years of lying at the bottom of moving water. They have a high likelihood of exploding when the heat of a fire pit causes the water inside the rocks to combust.
It’s dangerous to use river rocks in a fire pit for this reason.
6. Pea Gravel
Pea Gravel is made of small round rocks with the consistency of river rocks. These tiny rocks are too small to keep even heating.
They are also prone to popping when they become overheated. Small amounts of water in porous pea gravel rocks can cause them to pop and explode, turning into mini projectiles.
7. Basalt
Basalt is another common rock. It forms from a variety of materials that can include labradorite, augite, iron ore, olivine, or quartz.
It is a volcanic rock formed when these materials compress under tremendous heat. It is dark-colored, usually gray or black, and featureless.
It is a porous rock that has a high potential for storing water in the interior. This increases the chances of the rock exploding when exposed to high temperatures.
8. Soapstone
Soapstone is a metamorphic rock that is also called steatite or soaprock. It is composed of talc as the main material with chlorite, amphiboles, carbonates, micas, and other minerals.
It occurs through dynamo thermal metasomatism and metamorphism. This rock type forms near tectonic plates over thousands of years. It is a soft stone that is generally gray, green, brown, blue, or in various colors combined.
The stone has a soapy feeling and has a soft composition. Soapstone is not hard enough to be used for fire pits as it is easily damaged.
The porosity makes it prone to retaining liquid on the inside, making it prone to explosions when exposed to extreme heat.
9. Conglomerate
Conglomerate rocks are easy to identify by their multi-rock appearance. These clastic sedimentary rocks contain a variety of different rock types. The mixture may include sedimentary, metamorphic, or igneous rock fragments.
Large classes of rocks bind together mixed with sand, mud, silt, or other ingredients. They form in water where strong currents carry rock fragments and deposit them together over time.
These rocks are coarse-grained and range from soft to hard, with variations within the rock itself. Since these rocks form over centuries in bodies of water, there is a high likelihood that they contain water on the interior.
This makes them prone to explosion when exposed to extreme heat.
10. Metamorphic Rocks
Any rock classified as a metamorphic rock is not suitable for use in a fire pit. These are rocks that have changed from one type to another.
They commonly have water on the interior. Additionally, their composition or structure contains weaknesses that make them subject to uneven heating and expansion, causing the rock to split and explode.
You can find a variety of metamorphic rocks in the wilderness. They come in all shapes and sizes, and they are volatile and unsafe for fire pit use.
Helpful Tips To Know Which Stones Should Not Be Used In A Fire Pit
When sorting through the various stones available for building a fire pit, take your time to examine the properties of the rocks. Use the knowledge you gained here to determine which stones not to use. Here are four tips to keep in mind.
- Porous stones are more likely to contain water. These stones should not be used to build a firepit
- Stones collected from riverbeds, such as river rocks pose a risk of explosion when heated. They are identified by their smooth edges and surfaces
- Conglomerate stones that contain multiple types of rocks compressed into one rock may hold water. They may also be prone to cracking and splitting when exposed to heat.
- Gravel does not hold heat well. Pea Gravel is prone to popping and causing injury when it is heated. Avoid small rocks because there is no real benefit to using them.
Final Thoughts
Choosing stones for your fire pit requires careful thought and attention to avoid potential dangers and disappointment.
It’s wise to begin the selection process with a firm knowledge of which stones not to use for a fire pit. Small rocks such as pea gravel do not hold heat and pose a danger of becoming hot and popping out of the fire pit.
Soft rocks are likely to split and break. They are the most likely to contain water on the inside, a potential accelerator for breakage, and dangerous explosions for anyone standing nearby.
Examine the rocks visually to look for signs of porosity. If they are porous or soft, they’re not good choices for a fire pit. If the stones consist of materials that will crack or split when heated, avoid them.
Rocks that form in water are also volatile. Any rock of low density is subject to crack, explode, or be poor at retaining heat. These are the stones that should not be used in a fire pit.