Geologists define a rock as an aggregate of one or more minerals, or a body of undifferentiated mineral matter. Granite, basalt, limestone and sandstone are common examples. Here is a useful comparison: if a chocolate chip cookie were a rock, the flour, sugar, butter and chocolate chips would be the minerals that make it up. Minerals matter because they are the starting ingredients in just about every part of daily life, from smartphones to roads, homes and even food. What a rock is made of, the shapes of its grains and how the grains fit together are clues that let geologists read the rock's history, much as a reader works out a story from its pages.

Not every natural solid is a mineral, because a mineral must pass five tests. It forms naturally, so steel is not a mineral; it is inorganic; it is usually solid at room temperature; it has an ordered internal structure that generally forms crystals; and it has a definite chemical composition that may vary only within limits. Most minerals are compounds of two or more elements. But some elements, including gold, copper, platinum, sulfur and iron, occur naturally as minerals. Silicate minerals make up most of the rocks we see at Earth's surface. Geologists name six common minerals as examples: quartz, feldspar, mica, amphibole, olivine and calcite.

To tell minerals apart, geologists test how hard they are. In 1812 the German mineralogist Friedrich Mohs devised a scale, choosing ten minerals from very soft talc to very hard diamond. The scale ranks minerals by what can scratch what. A fingernail has a Mohs hardness of about 2 to 2.5, and a copper sheet is about 3. So a fingernail can scratch mica, which is only 2.5, but it cannot scratch quartz, which has a hardness of 7.

Carbon shows how much structure matters. Graphite and diamond are the two mineral forms of carbon, yet graphite is extremely soft while diamond is the hardest known natural substance. In diamond, each carbon atom is joined to four others by strong bonds in a tight, even pattern. You may have heard that diamonds form from coal, because both contain carbon. In fact, geologists say that is not true.

Igneous rocks begin as melt. Magma rises because it is hotter and lighter than the surrounding rock. What happens next depends on where it stops. If it reaches the surface it becomes lava and cools quickly into fine-grained rock. But if it stays below, slow cooling over thousands or millions of years lets the crystals grow large. Rock of this second kind, called intrusive, has a coarse texture, and granite, diorite, gabbro and peridotite are examples. Fast cooling leaves crystals little time to grow, so extrusive rocks are fine-grained or glassy.

Pumice shows what trapped gas can do. Some magmas hold several percent dissolved gas by weight under pressure. When they erupt, the gas bursts out and the froth that flies through the air solidifies into pumice. Basalt is a dark, fine-grained rock that usually forms from lava flows. Basalt underlies more of Earth's surface than any other rock type, including most of the ocean basins. Most of it formed in three settings: oceanic divergent boundaries, oceanic hotspots, and mantle plumes and hotspots beneath continents.

Basalt builds big mountains. Shield volcanoes are made of thousands of fluid basalt flows that spread far and cool as thin sheets, so they have broad, gently sloping sides. As of November 2026, Mauna Loa, the largest shield volcano, rises 13,677 feet above sea level. Its top is over 28,000 feet above the deep ocean floor. On Mars, Olympus Mons is a basalt shield volcano about 375 miles across and 15 miles high. As of November 2026 it is the largest known volcano in the solar system. In Northern Ireland, the Giant's Causeway is a lava flow that cooled and cracked into columns of basalt.

Sedimentary rocks are built from fragments of older rocks or from the remains of once-living things. They build up as deposits on Earth's surface and often show layers, or bedding. Five processes make them: erosion, weathering, dissolution, precipitation and lithification, where the first three make the raw material and the last two build new rock. In lithification, clay, sand and other sediments on the bottom of the ocean are slowly compacted into rock by the weight of the sediments above. Coal compacts a lot: about ten feet of plant debris becomes only one foot of coal, and the debris builds up very slowly.

Limestone, a sedimentary rock made mostly of calcite, usually forms in clear, calm, warm, shallow seas. That lets geologists reason backward: if limestone is found on land, the area was probably once under water. Chalk is a soft limestone made from the calcium carbonate shells of microscopic sea organisms. Fossils can tell even more, because Tibetan sandstones hold fossils from when the Tethys Sea covered the region. Those fossils show their geologic age and the environment and climate in which they formed. A rock can therefore hold a record of an ancient sea, long after the water has gone.

Metamorphic rocks form when rocks are squeezed and heated deep in the Earth or where tectonic plates meet. They do not melt. Instead, metamorphism turns the rock into denser, more compact rock and creates new minerals. Foliation is the striped look that forms when pressure squeezes flat minerals so they line up. Granite can turn into gneiss this way, as its long, platy minerals shift to point the same way and are squeezed into flat sheets.

Some metamorphic rocks have no stripes. With enough heat and pressure, limestone, sandstone and bituminous coal can become marble, quartzite and anthracite coal. Marble is often the host rock for gem minerals such as corundum and spinel. Even glacier ice counts as a rock: it is made of one mineral, ice, the crystalline form of water. Most of it forms by metamorphism, as thousands of snowflakes recrystallize into larger, denser ice crystals.

Dating old rock is hard, because Earth's oldest rocks have been recycled and destroyed by plate tectonics. Scientists cannot find the exact age of the Earth directly from Earth rocks. Radiometric dating measures the age of rocks from the decay of long-lived radioactive isotopes found in rocks and minerals. A parent isotope loses particles from its nucleus and becomes a daughter isotope of a new element. A half-life is the time for half of a sample's parent atoms to decay. The oldest dated Moon rocks are between 4.4 and 4.5 billion years old.

Rocks that formed in one place may have been altered or carried far from where they started, because the Earth is so old. Mountains tell the same story. The Himalayas began forming 40 to 50 million years ago when India and Eurasia collided. Both landmasses have about the same rock density, so neither plate could sink under the other. The Appalachian Mountains formed at plate boundaries long ago. Now the nearest boundaries are over a thousand miles away, so the landscape is wearing down.

Words to know

• aggregate: grouped mass
• lithification: turning into rock
• foliation: mineral striping
• isotope: atom version