Rocks and Minerals

The hard treasures under our feet

Rocks are everywhere. They make up mountains, beaches, the ground in your garden and even the stones in a path. But have you ever wondered what rocks are actually made of? The answer is minerals. To understand the world beneath our feet, we first need to know the difference between a rock and a mineral.

What is a mineral?

A mineral is a solid material that forms naturally in the ground. Every mineral has its own special "recipe" of ingredients and its own pattern, which means its tiny building blocks always line up in the same neat way. Because of this pattern, many minerals grow into beautiful shapes called crystals.

Here are some minerals you might already know:

• Quartz — a hard, often clear or white mineral found all over the world.
• Salt (its mineral name is halite) — the same salt you sprinkle on chips comes from the ground or from the sea.
• Gold — a soft, shiny yellow metal used in jewellery.
• Graphite — a soft, grey mineral. The "lead" inside your pencil is really graphite.

Each of these is a single, pure material with its own properties.

What is a rock?

A rock is made of one or more minerals stuck together. Think of it like a cake. A mineral is a single ingredient, like flour or sugar. A rock is the whole cake, made by mixing ingredients together.

For example, the rock granite is made of at least three minerals mixed together: quartz, feldspar and mica. If you look closely at a piece of granite, you can often see different coloured speckles. Each speckle is a different mineral.

The three families of rock

Geologists, the scientists who study rocks, sort all rocks into three big families based on how they formed.

Igneous rock forms when hot, melted rock cools down and hardens. Deep underground this melted rock is called magma, and above ground it is called lava. Granite and basalt are igneous rocks.

Sedimentary rock forms when tiny pieces of sand, mud, shells and old plants settle in layers, usually at the bottom of a lake or sea. Over a very long time, the layers get squashed and cemented into solid rock. Sandstone, limestone and chalk are sedimentary rocks. These rocks often contain fossils.

Metamorphic rock forms when an existing rock is changed by strong heat and pressure deep inside the Earth. The rock does not melt, but it is squeezed and baked into something new. Marble and slate are metamorphic rocks.

These three families do not stay the same forever. Over millions of years, one type of rock can slowly turn into another. That amazing journey is explained in the rock cycle.

Minerals we use every day

Rocks and minerals are not just interesting to look at — we could not live our modern lives without them.

• Salt flavours our food and helps keep us healthy.
• Iron, dug from the ground as an ore, is turned into steel for cars, bridges and buildings.
• Copper is pulled into wires that carry electricity around your home.
• Quartz and other minerals are used in computers and phones.
• Graphite lets you write with a pencil.
• Limestone is heated to make the cement that holds buildings together.

Almost everything humans build starts with a rock or a mineral. The same deep Earth processes that make these materials also shape mountains and oceans, and even help build the seabed where you can read about oceans and sea life.

How geologists identify a mineral

When geologists find a mineral, they run tests to work out what it is. You can think like a geologist using these clues:

• Colour — but be careful, because the same mineral can come in different colours.
• Shine (called lustre) — is it sparkly like metal, glassy, or dull?
• Hardness — can it be scratched easily, or does it scratch other things? Geologists use a list called the Mohs scale, where talc is the softest at 1 and diamond is the hardest at 10.
• Streak — the colour of the powder a mineral leaves when rubbed on a rough tile.

Try it yourself: a rock and mineral test station

Set up your own geology lab with a small collection of rocks and pebbles. Ask permission before taking any.

1. Lay out 5–10 different rocks on a tray.
2. Look: Use a magnifying glass to spot different coloured speckles. Speckly rocks are made of several minerals.
3. Hardness test: Gently try to scratch each rock with your fingernail, then with a coin, then with a steel nail (ask an adult). A fingernail scratches very soft minerals; a coin scratches medium ones; a nail scratches harder ones. Sort your rocks from softest to hardest.
4. Record: Draw each rock and write down its colour, shine and hardness.

You have just done real geology, using the same kinds of tests that scientists use to read the story of every rock.