Pick up any stone from a path and you are holding a small archive. Its grains record whether it cooled from molten rock, settled out of water or was squeezed deep underground. To read that archive, geologists start with the building blocks of rock: minerals.

What counts as a mineral

A mineral is a naturally occurring, inorganic solid with a definite chemical makeup and an orderly crystal structure. Each part of the definition matters. Ice in a glacier qualifies: it is natural, solid and made of water molecules locked in a regular pattern. Window glass does not, because it is made by people, and even natural volcanic glass, obsidian, fails because its atoms are jumbled rather than ordered. Coal fails too, because it is made from the remains of plants. Scientists have recognized around 6,000 minerals, yet a few dozen make up nearly all common rocks.

The reason is chemistry. Oxygen and silicon are the two most common elements in Earth's crust, and they bond together in a sturdy four-sided unit. Silicate minerals built from that unit, such as feldspar and quartz, make up most of the crust. Iron, aluminum, calcium, sodium, potassium and magnesium fill out the list of major elements.

Telling minerals apart

Color is the least reliable clue, because tiny impurities can tint the same mineral many shades. Geologists look instead at luster (how a surface reflects light), cleavage (whether it breaks along flat planes), density and hardness. In 1812 the German mineralogist Friedrich Mohs proposed a simple scale. The Mohs scale ranks hardness by scratching: a mineral that scratches another is harder than it. Talc sits at 1, quartz at 7 and diamond at 10. The scale is about order, not equal steps, so diamond is far more than ten times as hard as talc.

Another trick is the streak test: rub the mineral on unglazed porcelain and look at the color of its powder. The streak is much more consistent than the color of the lump. It also unmasks a famous impostor. Pyrite, or fool's gold, leaves a greenish-black streak, while real gold leaves a yellow one. Many hopeful miners learned this the hard way.

Igneous rocks: born from fire

Igneous rocks form when molten rock cools and crystallizes. Underground it is called magma; once it erupts it is called lava. The speed of cooling controls the texture. Magma that cools slowly deep underground grows large crystals, as in granite, while lava that cools quickly at the surface forms tiny crystals, as in basalt. Cool lava fast enough and no crystals form at all, which is how obsidian gets its glassy look. The ocean floor is made mostly of basalt; the continents are built largely of granite and similar rocks.

Sedimentary rocks: layers of history

Sedimentary rocks form at the surface. Some are made of fragments of older rock that are carried, dropped, packed down and cemented: sand becomes sandstone, mud becomes shale. Others form chemically, like rock salt left by evaporating seas. Others are made by life. Limestone is often built from the shells and skeletons of sea creatures. Because sedimentary rocks form at ordinary temperatures, they preserve what was buried in them. Nearly all fossils are found in sedimentary rocks; the heat that forms other rocks usually destroys remains. Sedimentary rocks cover most of the land surface, though often only as a thin skin over other rock.

Metamorphic rocks: changed without melting

When rocks are buried deep, caught in a collision between continents or baked beside a body of magma, they change. Metamorphism is the change of a rock's minerals and texture by heat and pressure without melting it. Shale becomes slate, then schist; limestone becomes marble; granite can become banded gneiss. Pressure often lines minerals up into layers, giving metamorphic rocks their striped or flaky look.

The rock cycle

These three families are not separate worlds. The rock cycle describes how any rock type can be turned into any other over time: igneous rock weathers into sediment, sediment hardens into sedimentary rock, buried rock is metamorphosed, and deeply buried rock can melt and become magma again. The cycle is powered by two engines. Earth's internal heat drives melting, mountain building and burial, while energy from the Sun drives the weather, rain and rivers that wear rock down. There is no fixed order: a granite can be lifted and weathered without ever becoming metamorphic, and a marble can be melted without ever becoming sediment. Hutton, whom we met in the previous level, was among the first to see rocks as part of a cycle rather than a one-way story.

Worked example: the life of a grain of quartz

Imagine a quartz crystal that grew slowly in granite deep beneath a mountain range. Over millions of years the mountains are lifted and worn down, and the granite is exposed. Rain and frost loosen the crystal; because quartz is hard and resists chemical attack, it survives as a grain of sand. A river carries it to the coast, where it joins a beach. The beach is buried, the sand is cemented, and our grain becomes part of a sandstone. Later, two continents collide; the sandstone is squeezed and heated into quartzite, a metamorphic rock. If that rock is dragged deep enough, it may melt and the cycle begins again. The same journey explains Michelangelo's David: Carrara marble began as limestone made of sea creatures, and was later transformed by the heat and pressure of mountain building in Italy.