Much like the creation of the earth itself, every vivarium starts with a single rock. Granted there are exceptions, like bare tanks, for example. Vivarium Rocks come in a huge range of possibilities, where some are used more noticeably than others.
They can scale as large as boulders to form rockscapes… To as tiny as clay minerals mixed with organic matter to form substrates. This definitive guide will provide an in-depth understanding of stones used in all types of enclosures as well as present a complete list of suitable rocks.
Rocks are an overlooked yet vital part of a vivarium’s ecosystem. Many hobbyists, like myself, start out building enclosures without giving much thought to what stones should go into a tank. We will often add them later, thinking aesthetically.
This is mostly due to a number of reasons. Lack of knowledge in the understanding of stones… And the fact that rocks are viewed as heavy accessories, instead of an essential foundation to build around.
What Are Vivarium Rocks?
Vivarium Rocks are various stones suitable for use in an aquarium or terrarium. Stones that form in nature, as well as artificially constructed rocks, are both popular sources for hardscapes, backgrounds, and substrates. Rocks are generally categorized by their formation and classified by their mineral composition.
It is always good to practice starting a build with Vivarium Rocks as the base material. Then pick substrates that pair well with the stones. Once the tank is established, choose plants and animals that are suitable for that environment.
If you are building an animal-specific enclosure then consider the needs of that pet and figure out the type of rocks that should coexist with them and build from there. This will assure all needs are met from the very beginning and make things like water parameters a lot easier to maintain in the long run.
Before diving into an in-depth list of suitable rocks to consider for any type of vivarium, I would like to first cover an important aspect of rocks that will give insight into how they work in conjunction with one another… This will make hardscape or substrate choices easier to construct as you plan your build.
Vivarium Rock Size
Building a bioactive vivarium takes a lot of time and an understanding of how different substances work together. Looking at things from the perspective of scale is key to all successful designs. Vivarium Rocks are no exception in this case.
The best way to group rocks is by their size. This section will provide a breakdown of how stones are grouped by physical proportion.
The chart above is based on the Wentworth scale. It defines the classification of a rock based on its grain size and the technical name of the group they are referred to when there are multiple rocks of the same size piled together.
This will aid in understanding how a vivarium should look when the design is based on what would form in nature. The three major divisions are gravel, sand, and mud.
Gravel
Any Vivarium Rocks larger than 2 millimeters are going to be considered gravel. This pile consists of boulders, cobbles, and pebbles. In most cases, boulders and cobbles will often be used as centerpieces for hardscapes depending on the size of the vivarium.
Pebbles and, on occasions where tanks are large enough, cobbles will function better as substrates or soil additives.
Boulders are typically any rock that measures 256 millimeters or greater. These are the most common types of Vivarium Rocks to use as hardscapes or foundations to mount plants and other hardscaping materials. Cobbles range between 255 and 64 millimeters in measurement. These sizes are suitable as hardscapes in nano tanks or mini vivariums.
Pebbles are usually what comes to mind when many think of gravel in an enclosure. They range in size between 63 and 2 millimeters.
Any variety of Vivarium Rocks between the cobble and pebble sizes are more than likely going to be used within the substrate area. That would include the drainage layer, the top layer, or the substrate entirely.
Sand
Sand-size grains are the next size down for Vivarium Rocks and are often used as substrates or additives to a soil mix. The term “sand” is commonly referred to as one specific type of mineral that many will associate with the soil found on beach shores.
Technically this can be any type of rock that ranges in the measurements of the classes: coarse sand, medium sand, and fine sand.
Coarse sand is stones that range between 2 and 0.5 millimeters in grain size. One could consider the subcategories of very coarse sand and coarse sand between this range as well. Medium sand is grain sizes between the measurements of 0.5 and 0.25 millimeters.
The smallest Vivarium Rocks in this pile are referred to as fine sand and will range between 0.25 and 0.06 millimeters. This is the tiniest of stones still visible to the naked eye when viewed as an individual particle. A subcategory of fine sand and very fine sand could be created for this pile as well.
Here’s a definitive guide on terrarium sands if you want to know more about sand.
Mud
Vivarium Rocks that fall into the mud group are typically not used in the hobby anymore due to the maintenance requirements needed to maintain such a fine matter. Mud-sized stones will often be used just as a substrate additive in small amounts or in composition with larger sedimentary stones. This pile consists of silt and clay.
Silt will range in the measurements of 0.06 and 0.004 millimeters. Subcategories for this grain size can further be broken down into coarse silt, medium silt, fine silt, and very fine silt. In water, these rocks will appear as an unsightly tannin typically light in color if contents are interrupted.
Clay is the absolute smallest of Vivarium Rocks. The measurements will range between 0.004 and less. This grain size will work best as a slight additive to substrates and will appear as a leaching tannin if added to moving water.
Types Of Vivarium Rocks
Now that you have a better understanding of how to group Vivarium Rocks based on their size, it’s time to explain what happens when they combine to make larger stones. Rocks are often classified by the minerals that make up the stone and can generally be placed in one of three categories based on how they form: sedimentary, metamorphic, and igneous.
Various types of stones from any of these categories can be suitable for an enclosure. It mainly comes down to the mineral composition when it comes to deciding which rocks are ideal for a specific type of tank. This section will provide a complete list of safe Vivarium Rocks one would consider for an aquarium and/or terrarium.
Sedimentary Rocks
Sedimentary rocks are stones that take formation as a result of mineral grains accumulating and cementing together. These types of Vivarium Rocks are usually more brittle and proper consideration should be taken on choosing stones that won’t dissolve quickly or crumble easily.
Their formation can occur in a number of ways, Further breaking this category down into subcategories: clastic, organic, and chemical.
Clastic sedimentary rocks form as a result of the lithification of sediments. I like to think of the word “plastic” when I see clastic because the composition of these rocks is typically not organic.
Elements like moving water and wind attribute to the formation of these types of stones. River streams and ocean tides are common examples of how these stones are created.
Organic sedimentary rocks take form as a result of organisms decomposing or manually cementing these stones together. This category can also be referred to as biochemical sedimentary rocks as well depending on the formation.
Examples of this process in nature are through shells and exoskeletons lithifying or corals building reefs out of calcium.
Chemical sedimentary rocks form as a result of dissolved sediments left behind water that has either evaporated or precipitated. Examples of this process taking place in nature are the cone-shaped rocks that cover the ceilings in caves. These are the more unusual types of Vivarium Rocks used in the hobby.
Below is a complete list of sedimentary rocks suitable for both aquariums and terrariums:
Sandstone (Quartzarenite Rock)
Sandstones are clastic sedimentary rocks composed of silica minerals. This variation of sandstone consists of mostly quartz and a small percentage of clay matrix that binds the minerals together.
It is a coarse-grained stone with a rough, slightly porous texture. These Vivarium Rocks come in a range of sizes and colors. This type of sandstone breaks into fractures and can easily be carved to form shapes.
Quartzarenite rock is inert and won’t cause water hardness/PH to change. It can be used as a rockscape, and background, and be broken down into the substrate. Ideally, sandstones can be used in soft water or freshwater environments where PH levels are required to stay under 7.
Strata Rock (Sandstone)
Strata Rock is a unique variation of sandstone that resembles stratum patterns found on weathered rocks in nature. These rocks virtually have the same type of composition as traditional sandstones with the exception of their surface texture.
Strata rocks typically come in boulder sizes and have a reddish-brown color. This type of sandstone breaks into fractures but by doing so could run the risk of damaging the aesthetic surface.
Like many arenite stones, strata rock is inert and won’t cause water hardness/PH to change. It can be used as a rockscape, and background, and be broken down into the substrate. They can safely be used in saltwater as well as freshwater environments where PH levels are required to stay under 7. Check out this extensive guide about Strata Rock (Sandstone) for more information about this type of stone.
Pagoda Rock (Sandstone)
Pagoda Rock is a unique variation of sandstone that resembles stratum patterns found on weathered rocks in nature. These rocks virtually have the same type of composition as traditional sandstones with the exception of their surface texture.
Pagoda rocks typically come in boulder sizes and have a reddish-brown color. This type of sandstone breaks into fractures but by doing so could run the risk of damaging the aesthetic surface.
Like many arenite stones, strata rock is inert and won’t cause water hardness/PH to change. It can be used as a rockscape, and background, and be broken down into the substrate. They can safely be used in salt water as well as freshwater environments where PH levels are required to stay under 7. Check out this extensive guide about Pagoda Rock (Sandstone) for more information about this type of stone.
Petrified Stone (Fossil Wood)
Petrified stone, also known as petrified wood or fossil wood, is a rock that takes the form of a decomposed plant. This stone is referred to as a chert which is a type of chemical sedimentary rock. The composition is mainly quartz mixed with a variety of many other minerals.
Due to the impurities lithifying with the quartz mineral, fossil wood can exhibit a variety of colors.
Petrified wood is an inert stone that will have little influence on water parameters. It is suitable for both freshwater and saltwater environments. Small amounts of metal will leech from this rock over a period of time making it ideal for planted tanks. Check out this extensive guide about Petrified Stone (Fossil Wood) for information about this type of stone.
Limestone (Calcarenite Rock)
Limestone forms in a number of fashions. One of the more common versions great for aquariums is calcarenite rock, also referred to as calcite sandstone or clastic limestone.
Carboniferous limestone is a sedimentary rock that is composed of mainly calcite minerals. This stone comes in a variety of colors due to impurities but most range between gray, yellow, and white.
Carcarenite rocks are composed of calcite-rich minerals that will raise water parameters. These should only be used as vivarium rocks in tanks that require a PH level above 7. Hardwater tanks thrive in conditions limestone rocks produce. Invertebrates and coral will benefit as well from the moderate amounts of calcium that will leech from this rock. Check out this extensive guide about Limestone (Calcarenite Rock) for more information about this type of stone.
Seiryu Stone (Mini Landschaft)
Seiryu stone is a unique version of calcarenite rock that is found in Japan. The properties of this stone are virtually the same as any other calcite-rich sandstone. Visually, this rock has a jagged appearance and resembles a miniature mountain… Hence the nickname “Mini Landschaft”. The rock’s color is typically a grayish-blue and some will even exhibit streaks of white.
This rock will raise water hardness and PH levels. Ideally, Seiryu Stone should go in hard water tanks where PH is Preferably above 7. On the contrary, Many hobbyists will use this rock in freshwater setups and buffer its effects of it with driftwood and moss. Check out this extensive guide about Seiryu Stone (Mini Landschaft) for more information about this type of stone.
Elephant Skin Stone (Dolomite Rock)
Dolomite rock is a limestone that has been modified through magnesium-rich groundwater. When the stone rises above ground, it is exposed to elements like wind and rain and begins to weather away. This results in a surface that’s aged, closely resembling the skin of an elephant.
Elephant skin stone is a more tolerable rock to weather in acidic conditions than most limestone. It can be used in freshwater setups but will require water buffering materials to keep the PH under 7. Furthermore, this rock is very suitable for hard water enclosures and saltwater tanks. Magnesium and calcium will leech equally benefiting aquatic life.
Shale Rock (Claystone)
Shale rock is a fine-grained stone composed of flaked clay minerals and quartz fragments. This stone falls under the clastic sedimentary rock category and is commonly referred to as Claystone. The rock is found in a variety of colors and patterns depending on the impurities cemented along with the clay during formation.
Shale exhibits a flat, narrow-like formation and will typically break off into sheets due to the platy clay minerals that make up the stone.
Claystone is generally an inert rock making it ideal for all types of vivariums. Many hobbyists will use this stone in freshwater setups since it is clay-based and has no impact on water parameters. Some claystone carries small amounts of calcite so always test for alkaline before using these in soft water tanks. Check out this extensive guide about Shale Rock (Claystone) for more information about this type of stone.
Ohko Stone (Dragon Stone)
Ohko stone is a clastic sedimentary rock that is composed of mostly clay minerals and bits of organic matter. Commonly referred to as dragon stone, this rock exhibits a scale-like appearance and is greenish-brown in color. This rock is a unique type of mudstone that is only found in specific parts of the world. Ohko stone has become an extremely popular stone to acquire in recent years.
Dragon stone is an inert rock that is well-suited for softwater aquariums. It is a considerably lightweight rock to work with and easily breaks apart to form smaller pieces. On the contrary, this stone can be rather hard to clean due to its many holes and porous texture.
Metamorphic Rocks
Metamorphic rocks are sedimentary rocks that are exposed to certain levels of heat and pressure simultaneously. As these rocks compress underground, higher temperatures will cause grains to crystallize, resulting in a new type of stone that is harder and more durable.
This is usually a safer alternative to consider for Vivarium Rocks if you’re a beginner hobbyist. This category of rock can be separated into two major subdivisions: foliated and non-foliated.
Foliated metamorphic rocks are stones that have a preferred orientation of aligned minerals. These rocks will typically break apart into thin sheets. This process of metamorphism can happen under a variety of stresses and is a result of platy minerals composing the majority of the rock.
Non-foliated metamorphic rocks are stones that do not have a preferred orientation of aligned minerals. These rocks will commonly break into fractures. This process of metamorphism will happen when there is no particular type of deferential stress when crystallization takes place.
Below is a complete list of metamorphic rocks suitable for both aquariums and terrariums:
Marble Rock (Metamorphic Limestone)
Marble rocks are what happens when limestone is compressed under heat. These rocks are referred to as either calcite marble or dolomite marble depending on the mineral composition of the stone before metamorphism. Marble will typically have a swirl pattern of pale colors like white, gray, and even pink.
This metamorphic limestone has a high level of hardness and can be rather dense in large amounts. Being calcite-dominated, I would recommend this stone be used in saltwater tanks that require levels of PH above 7.
This stone will increase hardness in the water so avoid using it in softwater enclosures. Vivarium rocks of this nature will weather slower due to their formation and are considered non-foliated metamorphic stones. Check out this extensive guide about Marble Rock (Metamorphic Limestone) for more information about this type of stone.
Slate Rock (Slate Stone)
Slate rocks are the result of a Shale that has undergone metamorphism. These stones are considered foliated as they tend to break into sheeted layers. Colors will usually range between different shades of gray and may exhibit reddish-brown in some areas.
This is attributed to metals like iron being oxidized at some point during formation.
Vivarium rocks like slate are generally inert stones in most cases. This makes them ideal for freshwater aquariums. When using foliated rocks like this type, be mindful of sharp edges before placing them around inhabitants. These stones are hard and have a moderate amount of density. Use them scarcely in glass enclosures that cannot tolerate huge amounts of weight.
Igneous Rocks
Igneous rocks are stones that form by crystallization from water or magma. These stones can evolve from either sedimentary or metamorphic rocks. Due to the extreme temperatures, these types of rocks undergo during formation, most impurities that would cause water parameters to fluctuate are gone.
Resulting in an inert rock safe for any type of vivarium. This category can be divided into two major subdivisions: extrusive and intrusive.
Extrusive igneous rocks are stones that form as a result of volcanic activity. The term volcanic igneous rocks are also commonly referred to in this category. Formation takes place when crystallization happens above the earth’s core. An example of this process would be lava cooling down after a volcano erupts.
Intrusive igneous rocks are stones that form as a result of magma cooling beneath the earth’s crust. This category is also commonly referred to as plutonic igneous rocks. As magma rises closer to the surface, the rocks it comes in contact with can undergo further metamorphosis causing them to become igneous in composition as well.
Below is a complete list of igneous rocks suitable for both aquariums and terrariums:
Granite Rock (Dimension Stone)
Granite rocks are the result of sandstones undergoing crystallization at the highest temperatures. This stone is referred to commonly as an intrusive igneous rock but it can form above ground in some instances.
The mineral composition of granite is mainly quartz as most other minerals won’t survive the extreme conditions this stone goes through during formation. These rocks are one of the hardest and heaviest on this list. Colors will range across the spectrum and the texture is typically coarse-grained.
This type of vivarium rock is generally inert and suitable for all types of tanks. PH levels and water hardness should remain neutral when using this stone in an enclosure. The durability of this rock will outlast many tanks along with inhabitants which is great for reuse. Check out this extensive guide about Granite Rock (Dimension Stone) for more information about this type of stone.
Obsidian Rock (Volcanic Glass)
Obsidian rocks are a type of volcanic glass that forms when magma is cooled very quickly. These stones can form above ground as well as below the earth’s crust close to volcanic vents. It is generally black in color and transparent when held to the light. The hardness of this rock is equal to glass. The texture is nonporous and composed of fine-grained minerals.
Volcanic glass is non-foliated and will produce sharp edges when broken. This can be extremely hazardous if placed around inhabitants. Furthermore, the rock can be polished smooth and one should consider doing so when using it in a vivarium. Check out this extensive guide about Obsidian Rock (Volcanic Glass) for more information about this type of stone.
Basalt Rock (Lava Rock)
Basalt rocks are igneous rocks that form as a result of cooling lava. Commonly referred to as lava rocks, these are the most abundant forms of volcanic rocks found today. Depending on the mineral impurities contained within the stone during formation, colors will range between black, gray, and reddish-brown.
This rock is considered non-vesicular and is less porous than any other type of lava rock.
This vivarium rock is inert making it great for both soft and hard water tanks. It will have no effect on PH/hardness levels. This is a good alternative to Scoria rocks if you want something non-porous, inert, and dark in color. Check out this extensive guide about Basalt Rock (Lava Rock) for more information about this type of stone.
Scoria Rock (Lava Rock)
Scoria rocks are extrusive igneous stones that form from cooled magma. They are also referred to as lava rocks as well as vesicular basalt rocks. These vivarium rocks trap gases during formation resulting in the heavily porous texture we find them in. Scoria is considered a non-foliated stone and can be found in the same colors as Basalt rocks.
This igneous rock is inert making it great for both soft and hard water tanks. Scoria has an excellent surface for plants and bacteria to latch to. I would strongly recommend this stone for planted tanks. It will have no effect on PH/hardness levels. This is a good alternative to basalt rocks if you want something porous, inert, and dark in color.
Pumice Rock (Floating Stone)
Pumice rocks are igneous rocks that form along with the outermost areas of basalt. This rock is considered an extrusive igneous rock because it typically takes formation during violent volcanic activity. It is commonly referred to as the floating stone because it has a density lighter than water.
These floating rocks eventually sink though once it becomes saturated with water, bacteria, or other sediments. The colors are usually light ranging between white and gray.
Like any lava rock, pumice is inert and can be used in both freshwater and saltwater tanks. This stone breaks easily into a pebble. It is frequently utilized as a substrate additive to retain moisture. Some hobbyists don’t like the look of the light-colored gravel scattered along with the soil and consider it unsightly. So take that into consideration before using it in a vivarium.
River Rocks (Riverstones)
River rocks can be any type of stone that is polished by moving water. Other common names include beach pebble and Riverstone because of the locations these stones are found after forming.
Some of the more popular types used in the hobby are mostly metamorphic or igneous due to the durability of those stones handling the tumbled formation better. These rocks range in any color depending on their mineral composition. The textures of Riverstone are usually smooth and can be either porous or non-porous.
River rocks and beach pebbles may or may not be inert. It comes down to the type of rock it converted from. I would recommend always buying these rocks from vendors who sell them exclusively for aquarium use to assure they won’t cause PH/hardness issues. If you are collecting these rocks from somewhere outdoors, before placing them in a softwater vivarium, always run a quick alkaline test.
In Summary
Vivarium Rocks are vital to an enclosure’s ecosystem. In a way, everything in a vivarium will either consist of rock material or benefit from it. Experiment with different sizes and formation types while remaining conscious about how those rocks will affect the enclosure and its inhabitants.
Acidic, clay-based stones and inert rocks are best for softwater tanks that need to keep the PH below 7. Alkaline-based stones heavily composed of calcite are ideal for hard water tanks that require PH levels to stay above 7.
I truly hope this geological approach to vivariums was stated in an easy-to-digest format and has provided value to your future builds. If there are any rocks not mentioned here that you would like for me to cover, feel free to let me know down below. Any particular rocks piqued your interest the most?
Frequently Asked Questions
Yes, you can put rocks in a vivarium. Rocks can provide a flat spot for your vivarium inhabitants to hide or sit on. When choosing rocks for your vivarium, it is important to make sure they are non–toxic and will not throw off the pH balance of the water. Large, smooth river rocks are an ideal choice.
To properly sterilize rocks for use in a vivarium, follow these steps:
- Fill a large pot or bucket with warm water.
- Add about a tablespoon of bleach per gallon of water.
- Soak the rocks for at least 10 minutes.
- Rinse the rocks several times with clean water.
- Let the rocks air dry before using them in the vivarium.