Rock: :

Noun [from the vulgar Latin rocca] The material that composes the mainly solid parts of the Earth and other celestial bodies (planets, moons, meteorites, comets). Rocks are made up of assemblages of one or more minerals, either crystalline or glassy. They are often more or less hard and coherent (stones, pebbles), but can sometimes be plastic (water-swollen clays) or loose (sands) (adapted from Foucault & Raoult, 2010, and Michel, 2016).

Geologists study petrology (from the Greek petra, stone, and logos, discourse) to describe the different rocks accessible to them (A).and classify them into families based on their formation (B).

A. Description

The description of rocks is also known as petrography (from the Greek petra, stone, and graphein, to write). This can be done either on hand specimens visible to the naked eye (macroscopic scale) or with magnifying tools (microscopic scale).
Rocks are described based on various features:

Although many rocks are solid, there are also loose rocks such as clays or marls, and rocks that are powdery, fibrous, etc.

Appearance can vary, even between two identical rocks. Factors like oxidation or biological growth (algae, lichen, etc.) can alter the outer color of a rock. This is why fresh fractures are preferred for observation whenever possible.

Even without breaking it, rocks may show natural break planes where they split easily. This type of splitting, or cleavage, can have characteristic forms (rhombic, rounded, thin plates, etc.).

A fracture shows how the rock breaks and also reveals its mechanical strength.

Depending on the minerals present (type, size, shape), the texture of a rock can range from rough to smooth.

Density is calculated by comparing the mass of a rock sample to the mass of an equal volume of water. One can also estimate relative density by hand-weighing, though this is much less accurate.

Hardness is a rock’s ability to scratch another material. It is measured on a scale from 1 to 10, known as the Mohs scale, with 1 being the softest and 10 the hardest. Note: hardness is different from strength.

Some rocks may have a distinct taste, especially those containing “salty” minerals like halite or sylvite. Porosity can sometimes be assessed if the sample sticks to the tongue.

Odors can also be diagnostic: for example, siliceous minerals may emit a distinctive smell when struck, while sulfur-containing minerals often smell like rotten eggs.

Rocks also contain many macroscopic and microscopic features that aid identification — such as minerals, faults, fossils, included rock fragments, and more.

A rock particle can already be named based on its size. Free particles resulting from rock breakdown are called

sediments

. If they are produced by volcanic activity, they are referred to as

pyroclasts

.

2. Classification

The synthesis of these macroscopic and microscopic observations — often supported by physico-chemical analyses — allows geologists to classify rocks into

three main families

:

1. Sedimentary Rocks

These are

exogenous rocks

(formed at the Earth’s surface). Sedimentary rocks form through the accumulation of sediments (fragments of biological remains like shells or minerals) and/or by precipitation from mineral-rich solutions. Their origins include:

• Detrital when the rock is formed by erosion, transport, and deposition of fragments from preexisting rocks (e.g., sandstone, conglomerate, sedimentary breccia, argillite).
• Biogenic when the rock is directly built by the activity of living organisms (e.g., biogenic limestone, chalk, coquina, stromatolite, coal).
• Physico-chemical when the rock forms through changes in environmental factors like pH, oxygenation, solubility, ionic potential, temperature (moderately), and pressure (moderately) (e.g., dolomite, flint, rock salt, travertine, calcareous concretions).

2. Igneous Rocks

These are endogenous rocks (formed, at least partially, at depth within the Earth). They originate from

magma

, with cooling speed and depth depending on the geological setting. Igneous rocks are categorized as:

• Volcanic when magma quickly reaches the Earth’s surface. Cooling is rapid (within hours or days), leading to the formation of volcanic glass (amorphous) along with some crystals. Examples: basalt, obsidian, andesite, ash, pumice.

• Plutonic when magma remains several kilometers below the surface and cools slowly (over thousands or even millions of years). Larger crystals have time to form, and are often visible to the naked eye. Examples: granite, gabbro, diorite.
  There are also intermediate rocks formed in dykes or sills that cool relatively slowly. Examples include dolerite and lamprophyres (e.g., kersantite, minette).

3. Metamorphic Rocks

Also endogenous, metamorphic rocks result from the transformation (metamorphism) of sedimentary, igneous, or pre-existing metamorphic rocks due to increased pressure and/or temperature. Occasionally, chemical inputs also influence metamorphism. This process creates new minerals and changes the texture and structure of the original rock. Examples: gneiss, schist, marble.