Natural stones can be classified to their scientific or commercial name.
The scientific classification is described in the European standards EN 12407 and prEN 12670. These standards have the disadvantage that they are only comprehensible by geologists and petrology experts, but the scientific classification does give an instant indication of the characteristics and the compensation of natural stone. Next to a scientific classification natural stone also has a commercial classification and specification. In the commercial classification the direct link between name and characteristics of the stone is sometimes no longer present.
E.g. the commercial classification ‘marble’ refers to all calciferous natural stones that can be polished, but according to the scientific classification the real marbles (metamorphic calciferous stones) and certain limestones (sedimentary stones) also belong to this group. The same applies for the commercial term ‘granite’ for hard, polished stones from both igneous and metamorphic origin.
Scientific classification
Scientifically we can distinguish three big groups of natural stones: the igneous, the sedimentary and the metamorphic stones. The three kinds of stones have clear differences in appearance and characteristics.
a. Igneous stones
Igneous stones are formed through liquid magma that cooled down and solidified in or on the earth’s crust. Depending on the location of hardening, three groups of igneous stones can be distinguished: extrusive stones, intrusive stones and minor intrusive stones.
- Extrusive stones
Igneous rocks formed by the cristallization of magma at the Earth's surface are called extrusive rocks. These stones are characterized by fine-grained textures because their rapid cooling at or near the surface of the Earth did not provide enough time for large crystals to grow. Sometimes air bubbles may appear on the surface of the stone. The most common extrusive rock is Basalt. - Intrusive stones
Igneous rocks formed by the cooling down of magma under big constant pressure at a depth within the Earth are called intrusive rocks. These stones have a course grained appearance with visible (large) crystals: the cooling down of magma deep in the Earth is much slower than the cooling process at the surface, this enables the crystals to grow larger. The most common intrusive rock is granite. - Minor intrusive rocks
Igneous rocks formed by the cooling down of magma under big constant pressure in minor intrusive bodies such as dykes, sills and volcanic plugs in the Earth are called minor intrusive rocks. The minor intrusive stones are formed quicker than the intrusive stones, therefore these stones have a fine to medium grain size. An example of a minor intrusive rock is a porphyry.
b. Sedimentary stones
The process of sedimentary rock formation involves a variety of sediments (tiny grains of sand, broken pieces of rock, fragmented seashells, layers of mud, plant or animal remains) carried by gravity, water, wind or other agents of weathering and erosion to a destination where they are deposited. These sediments usually deposit in layers (called strata) where they become cemented or compacted together over time, forming a solid sedimentary rock. Some slates are sedimentary stones.
Based on the origin of the sediments, sedimentary stones can be divided into the following categories:
- Detrital sediment stones are formed when solid fragments (weathered pieces of rock, sand, mud etc.) are released from the place of weathering, transported and finally deposited. E.g. sandstones and some limestones.
- Chemical sediment stones are formed out of direct chemical precipitates from materials dissolved in water (e.g. salt-deposits). These dissolved materials are released form the place of weathering and after transportation, deposited as chemical sediments. E.g. travertine.
c. Metamorphic stones
Metamorphic rocks are sedimentary or igneous rocks (or even metamorphic rocks) that have been altered by extreme heat and/or pressure (e.g. formation of mountains). During this process a physico-chemical alteration of the stone's elements takes place that can lead to mineralogical and textural changes. This process changes the characteristics of the original rock. Examples of metamorphic stones:
- Quartzite is a converted sandstone (sedimentary stone)
- Marble is a converted limestone (sedimentary stone)