What Is a Rock?

A rock is a natural solid material made up of one or more minerals. Almost the entire solid Earth is made of rocks: from the loose stones on a beach to the mountains, the cliffs, the deep ocean floor and the molten layer beneath your feet. Geologists divide rocks into three main types based on how they formed: igneous (from cooled magma), sedimentary (from compressed layers), and metamorphic (changed by heat and pressure). All three are constantly being recycled in a slow loop called the rock cycle that can take millions of years to complete.

  • Three rock typesIgneous, sedimentary, metamorphic
  • Minerals in average rock2 to 6 different onesMixed together
  • Most common element in crustOxygenApproximately 46% by weight
  • Most common rock-forming mineralFeldsparRoughly 60% of the crust
  • Oldest known rocksApproximately 4 billion yearsAcasta Gneiss, Canada
  • Oldest mineral grain4.4 billion yearsA zircon crystal from Western Australia

Rocks vs minerals

It is worth getting this distinction right.

  • A mineral is a naturally occurring solid chemical compound with a specific composition and crystal structure. Quartz, mica, feldspar, calcite and pyrite are all minerals. Over 5,000 are known.
  • A rock is a natural mixture of minerals stuck together. Granite, for example, is a rock made mostly of three minerals: quartz, feldspar and mica.

So minerals are the chemicals, rocks are the chemicals in mixed packages. Think of minerals as ingredients and rocks as the meals.

The three main rock types

  • Igneous: formed when hot molten rock (magma) cools and hardens. Examples: granite, basalt, obsidian, pumice.
  • Sedimentary: formed from compressed layers of sand, mud, plant remains and shells. Examples: sandstone, limestone, chalk, coal. Most fossils are found in sedimentary rock.
  • Metamorphic: formed when other rocks are changed by intense heat or pressure. Examples: marble (from limestone), slate (from shale), gneiss (from granite).

How to recognise a rock

Geologists look at several features.

  • Colour: tells you something about the minerals inside. Dark rocks often contain iron and magnesium; pale rocks often contain quartz and feldspar.
  • Grain size: tiny smooth grains usually mean fast cooling or fine sediment; large visible crystals usually mean slow cooling deep underground.
  • Hardness: can you scratch it with a fingernail? With a steel knife? With glass? Tells you which minerals are inside.
  • Texture: smooth, rough, layered, bubbly, glassy?
  • Weight: dense igneous rocks like basalt feel heavier than light pumice or chalk.
  • Fossils or layers: signs that the rock is sedimentary.
Fact The oldest individual mineral grain ever found is a tiny zircon crystal from the Jack Hills of Western Australia, dated at 4.4 billion years old. That is older than the oldest rocks (4 billion years) and almost as old as the Earth itself (4.54 billion years). The grain shows that solid crust must have formed on Earth incredibly soon after the planet itself formed.

Why rocks matter

Rocks are not just lumps in the landscape. Almost everything in modern life depends on them.

  • Soil: weathered rock is the basis of soil, which grows our food.
  • Building materials: bricks, stone, concrete, cement, plaster, glass all come from rocks.
  • Metals: iron, copper, aluminium, gold, silver, lithium, uranium all come from rocks.
  • Fossil fuels: coal, oil and gas all come from buried plant and animal remains in sedimentary rocks.
  • Energy: even nuclear and geothermal energy ultimately depend on the chemistry of rocks.
  • Records of the past: rocks preserve the history of climate, life and Earth itself, going back billions of years.
Did you know? The Earth's crust is mostly made of just eight chemical elements. Oxygen alone makes up roughly 46% of the crust by weight, silicon another 28%, then aluminium, iron, calcium, sodium, potassium and magnesium. Almost every rock you can see is built from these eight elements in different combinations.
Deeper dive: how every rock has a story

A geologist looking at a rock can often read a remarkable amount of history from it. Each feature is a clue.

The crystal structure tells you whether the rock cooled fast (small crystals) or slowly (large crystals). Slow cooling usually means deep underground; fast cooling usually means at or near the surface.

The mineral mix tells you the chemistry of the original magma or sediment, which often reveals what kind of environment the rock formed in.

The texture tells you whether the rock was once liquid (igneous), was deposited grain by grain (sedimentary), or was squeezed and folded under pressure (metamorphic).

Fossils tell you exactly which period of Earth's history the rock is from, and what life looked like at the time.

Layers and folds tell you how the rock has been moved since it formed: stretched, compressed, broken or flipped over.

Radioactive dating can pin down the exact age, often to within a few percent.

A single hand-sized chunk of rock can contain enough clues to tell its 100-million-year history: the volcano it came from, the sea it sat in, the mountains it was pushed up into, the river that carried it down, and the beach where it eventually ended up. Geology turns out to be a kind of detective work.

For more, see minerals and the rock cycle.