Origin of main types of natural stone: easy geological definition

We open our blog with basic and essential information about the main types of natural stone and we would like to do it with a simple and accessible vocabulary, where we will be able to quickly understand its geological origin, something that will definitely help us for a better understanding when we look at all different features and patterns of each particular stone.

There are 3 main types of rocks that are classified according to their origin. It means that it depends on their composition, but also on the process by which they were joined. Then, what is the geological origin of natural stones?

First of all, it is important to differentiate a mineral from a rock, as both are commonly referred to as stones. A mineral is a homogeneous chemical substance, and the combination of several minerals forms a rock.

In this way, several rocks can include the same minerals, but since they do not have the same formation conditions, the result is translated into rocks with different characteristics and, therefore, of different types.

This is a summary of the most commercialized natural stones, classified according to their geological origin.

1. Igneous (or magmatic)

1.1. Plutonic (or intrusive) – GRANITE, GABBRO (black granite)

1.2. Volcanic (or extrusive) – BASALT

2. Sedimentary

2.1. Detritic (or clastic) – SANDSTONE

2.2. Non-detrital (or chemical) – LIMESTONE, TRAVERTINE, DOLOMITE

3. Metamorphic

3.1. Laminated (o foliated) – SLATE, GNEISS

3.2 Non-laminated (or crystalline) – MARBLE, QUARZITE   

Following up, we detail a brief definition of each group of rocks, along with the most common types of natural stone that we usually find in the construction industry.

1. IGNEOUS ROCKS

They are produced by cooling the magma (a fluid formed by molten rock) coming from the interior of the Earth. They constitute 95% of the upper part of the earth’s crust, where they are mostly hidden by a layer of sedimentary and metamorphic rocks.

There are two types of igneous rocks:

1.1 PLUTONIC (OR INTRUSIVE)

Rocks solidify through the slow cooling of magma inside the Earth. This process originates clearly visible large minerals.

GRANITE

It is composed of feldspar, mica and quartz, with size grains visible on the surface. Depending on its mineral composition, it gives the granite a pink, red, grey or white color with dark mineral grains visible throughout the rock.

It is normally generated in continental arcs and the most important granite deposits are found in Spain, Egypt, India, South Africa, Argentina, China, Italy, the United States, Ukraine and Brazil.

GABBRO

It is the geological denomination, although in the industry it is commonly known as black granite.

It is a thick-grained rock. It is composed of ferromagnesian minerals (which provide dark colors) and contains only a few quartzes (less than 5%). It has tonalities that range from dark green to black. It is generated in the mid-oceanic ridges and the most important deposits of gabbro are found mainly on the African continent.

Both granite and gabbro are characterized by their strength and hardness. They withstand atmospheric alterations, wear and scratches and, in addition, they are insulators of humidity. This makes them ideal materials for any type of application in the field of construction, from exterior facade cladding to kitchen countertops

1.2 VOLCANIC (OR EXTRUSIVE)

When the magma sprouts (volcanic eruptions) to the earth’s surface, the rocks solidify through rapid cooling, not giving the mineral crystals time to grow, so they are not visible on the surface.

BASALT

It is a fine-grained stone, rich in magnesium and iron silicates. It is dark, black or gray in color and is one of the most abundant rocks in the earth’s crust since it forms most of the ocean floor. Despite its firmness, the fineness of its grain makes it polish naturally over time and with the time it becomes slippery, the reason why is not recommended for certain applications.

2. SEDIMENTARY ROCKS

They are formed by the accumulation of sediments that come from the weathering, erosion, transport and consolidation of sediments from any other type of rock already existing on the earth’s surface, whether they are igneous, metamorphic and even sedimentary rocks. This combination of processes are called diagenesis.

The accumulation of these rocks forms mainly in layers, also called stratum.

In turn, sedimentary rocks are divided into two groups:

2.1. DETRITIC (OR CLASTIC)

They are formed by accumulation of sediments from the erosion of rocks exposed to the surface. They are generated through the sedimentary cycle.

SANDSTONES

It is formed by grains of sand united by cement. With a medium grain size, sediments can be well appreciated on the surface.

It is composed of quartz, feldspars and clays. Sandstones rich in quartz are called quartz sandstones, those rich in feldspar are called arkose, and those rich in lithics are called lithic sandstones. If the proportion of calcium carbonate is important, they are called calcarenite.

Being composed of quartz, it is characterized by flexibility and resistance. It is mainly used for construction, facade cladding, and interior and exterior decoration.

2.2 NO DETRITAL (OR CHEMICAL)

They originate from the precipitation of dissolved minerals that are transported and concentrated in a basin, which allows their accumulation.

LIMESTONES

It forms predominantly on the seabed where material rich in calcium carbonate (calcareous material) accumulates. It can be formed by the effects of water precipitation (limestone, chemical or inorganic), segregated through the use of marine organisms, such as algae and corals (biochemical limestone), or it can be formed from the carbonate skeletons of marine organisms (bioclastic limestone). The sediment that composes it may have been derived from the dead remains of marine organisms (fossils) such as corals, microscopic algae or shells, among others.

It has a variable tonality, but we can usually find it in light colors, especially white.

Limestone stands out for its resistance to flexibility, impacts and compression. Although due to its cracks, it is permeable. As it is a soft and durable natural stone, it is frequently used in masonry, construction, decoration and facades.

TRAVERTINE

It is composed almost entirely of calcium carbonate, with some impurities of iron and magnesium in its composition that cause its banded appearance and can be found in various colors ranging from cream, grey, brown, yellow, red, and even green. Its main minerals are calcite, aragonite and in smaller amounts dolomite.

It is very similar to limestone in composition. They differ mainly in the geological formation environment. Limestone forms in marine environments, while travertine forms with the dissolution of rocks containing calcium carbonate in environments such as caves, caverns, or volcanic hydrothermal systems.

DOLOMITE

It is basically composed of the mineral dolomite, whose chemical composition is calcium and magnesium carbonate. Dolomite must contain at least 50% of dolomite and is considered pure when that percentage exceeds 90%. If it contains less than 50% it is a dolomitic limestone. It is a grayish rock, although it can also be white with gray or brown tones. Dolomite is often very similar to limestone, and the two are often indistinguishable in the field.

3. METAMORPHIC ROCKS 

When igneous or sedimentary stones are subjected to high pressures and temperatures, transformations occur in the minerals, forming these rocks through a process called metamorphism.

Some of the most common metamorphic rocks are the following: marble, quartzite, slate, gneiss, phyllite or schist.

Depending on the external appearance of these rocks, they can be classified into two main types:

3.1. LAMINATED (OR FOLIATED)

They are generated by mineral composition (mainly micas). As its name suggests, being minerals with a laminar shape, they generate layers that can easily separate and break.

SLATE

Composed mainly of clay, it separates into thin layers, mainly black in color, although it also exists in other colors. It has a shiny surface due to the presence of mica crystals and has a lower degree of metamorphism. Being waterproof and resistant to weather attacks, it is widely used to cover especially exteriors.

GNEISS

It is composed of the same minerals as granite (quartz, feldspar and mica), but differs in the orientation of the minerals forming bands and alternating layers of dark minerals (ferromagnesian) and light minerals (quartz and feldspar).

It is the rock that has a higher scale of metamorphism.

3.2 NON-LAMINATED (CRYSTALLINE)

They do not have layers and are characterized by breaking irregularly, without separating into layers. They are generally homogeneous and are composed of few minerals.

MARBLE

Composed mainly of calcium carbonate that has undergone crystallization processes, in most cases during the metamorphization of limestone and dolomite (carbonate rocks).

Thanks to this component, they are easy to polish and have a lot of shine.

Being a hard and white natural stone, they have been widely used for monuments and sculptures throughout history. Currently, its use is very extensive in decoration and cladding, mainly for interiors.

QUARTZITE

It comes from the metamorphism of quartz sandstones. It is formed as a product of the recrystallization of quartz, from a sandstone protolith, which is composed mainly of quartz clasts, and this is altered by the heat, pressure and chemical activity of metamorphism. Metamorphism recrystallizes the sand grains and the silica cement that binds them together. In this way, it constitutes a network of quartz grains linked together. In composition, most quartzites become more than 90% quartz and some even 99%.

We conclude here our first post. We hope it will help you to identify from now the different types of natural stones. In our next article, we will talk about other types of stone such as Onyx, Alabaster and other Semi-precious stones, which deserve a separate chapter.

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