Scientific and commercial breakdown
Natural stones can be divided according to their commercial or their scientific classification. The scientific name is described in the European standard NBN EN 12670 (Natural Stone – Terminology).
A major disadvantage of this standard is that many of the terms used are only understandable for geologists or petrologists. On the up side, the scientific classification does offer immediate insight into the properties and the composition of each type of natural stone.
Beside this scientific classification, there is also a commercial classification. The commercial denomination sometimes has nothing to do with the characteristics of the natural stone. The European standard NBN EN 12440 lists both the official commercial denomination and the scientific denomination of approximately 2400 types of natural stone. The standard does not concern itself with the commercial names of natural stones quarried outside Europe.
The standard sets forth that the commercial denomination must clearly refer to the region where the stone is being quarried, its geological nature and its color.
Magmatic rock or igneous rock
Magmatic rock is created by the cooling and solidification of liquid magma, beneath or on the surface of the earth’s crust. Depending on where this cooling process took place, we distinguish between three groups of magmatic rock : volcanic or extrusive rocks, gangue, and plutonic or intrusive rocks. Further classification of magmatic rock is based upon different criteria, such as the way it is formed (which defines the texture) and the mineralogical and/or chemical composition that defines the crystals that are present.
These rocks are generally hard, acid-resistant, frost-resistant and wear-resistant. Volcanic rock has solidified on the earth’s surface. Magma from the earth’s core may rise to the surface (lava). After, for instance, a volcanic eruption, magma will quickly cool and solidify. As a result, volcanic rocks have almost no visible minerals (very fine-grained texture). There is a possibility of air bubbles forming. Volcanic rock is usually uniform in appearance and composition. Basalt rocks belong within this group. Plutonic rocks have cooled down gradually and solidified under constant, high pressure, deep within the earth’s crust.
As a result, these rocks are much more coarse-grained, with crystals that are clearly visible. Examples of plutonic rocks are granite and gabbro. Gangues are formed in gaps or cracks in the earth’s crust, where they cooled slowly and under high, constant pressure.
They have solidified in less time than the more coarse-grained rocks. Texture and composition of gangues are quite regular, but there are important variations between the stones themselves. Some examples are diabase and porphyry.
Sedimentary rock was formed by the accumulation and settling of waste materials, residual, colloidal or dissolved matter, that were transported to the earth’s surface by water or wind. This matter is formed by the weathering and erosion of magmatic, metamorphic or earlier sedimentary rocks. The sediments were originally particulate matter (think of sand in the desert or on the beach) that compacted under the influence of various factors.
The main feature of sedimentary rock is that it is generally deposited in horizontal or parallel consecutive layers. The thickness and the composition of these layers may vary. The layers are clearly visible as they usually vary in color, composition, size of the grains and texture. Sedimentary rocks tend to break more easily along the surface of these layers.
Within the class of sedimentary rocks, we distinguish between the clastic rocks, biogenic rocks, organogenic rocks and evaporites. Clastic rocks are formed by accumulation of pieces of rock from the earth’s surface. Some examples are sandstones and some limestones. Biogenic rocks are formed by deposits of small organisms, such as corals and calcareous algae.
Organogenic rocks are formed by accumulation of organic particles, such as plants. For instance, detritus from plants become peat that, after various processes, is transformed into lignite and coal. Evaporite rocks are formed by the precipitation of minerals from saturated water. Examples include travertine, gypsum and anhydrite.
Metamorphic rocks are formed by subjecting sedimentary rock, igneous rock or another older metamorphic rock to high temperature and pressure conditions (for example, during the formation of mountains).
This process results in a change in the physical and chemical properties of the minerals, and sometimes a profound change in their mineralogical composition (formation of new materials) and their look (specific texture and structure). As such, the properties of the original rock change.
Examples of metamorphic rocks are gneiss – formed by the metamorphism of granite (magmatic rock), quartzite – formed by the metamorphism of sandstone (sedimentary rock), marble – formed by the metamorphism of limestone (sedimentary rock). These three types of rock are largely different in look and properties.
The commercial denomination may be very different from the scientific denomination or classification. Indeed, often it is not based upon the formation, the texture and the mineralogy. Instead, natural stones are divided into big families, such as marbles, granites, that have nothing to do with the scientific denomination marble or granite. Because of all this, the commercial denomination often does not reckon with the connection between the name and the properties of the stone.
For instance, the commercial name ‘marble’ stands for all calcareous natural stone that can be polished. In this group, we can find the real marbles as per the scientific classification (metamorphous calcareous rock), but also some marble-like limestones (sedimentary rock). The same is true for the commercial denomination granites, that may refer to hard and polished magmatic or metamorphic rock.