A rock is not just a lump. Every rock is made of minerals. A mineral is a natural solid with a set chemical makeup and an orderly crystal structure. Many minerals grow as crystals. Look at a rock and you may see many of them. Look closely, and you can see the minerals that make up the rock.

Minerals can be soft or hard. The Mohs scale ranks how hard minerals are by what can scratch what. Talc is a very soft mineral. Quartz has a Mohs hardness of 7. Diamond is the hardest natural substance known. So the range runs from the softest minerals to the hardest of all.

There are three main kinds of rock: igneous, sedimentary and metamorphic. Each kind is made in a different way. Hot melted rock cools, and it hardens into solid rock. Igneous rocks that cool inside the Earth are called intrusive. Those that cool at the surface are called extrusive.

The speed of cooling shows in the crystals. Rock that cools slowly deep underground grows big crystals. Lava that cools fast grows tiny ones. Granite is a rock with crystals big enough to see. Obsidian is a kind of volcanic glass. Both began as hot melted rock.

Wind and rain slowly break big rocks into smaller pieces. Sedimentary rocks are made from bits of older rocks and from living things. Sandstone is a rock made of sand grains stuck together. These rocks often form in layers, one on top of the next. Each layer is a page in the story of the land.

Most limestone forms in the sea. Some of it is made from the shells and skeletons of sea animals. Coal is a rock that comes from plants that lived long ago. So living things can end up as stone. Their long story is kept in the rock.

Great heat or pressure can change rocks, too. Rocks changed this way are called metamorphic. Shale, a soft rock, can become slate. Marble is a rock made when limestone is changed by heat and pressure. Gneiss has stripes, or bands.

The rock cycle is the never-ending change of rocks from one type into another over geologic time. Metamorphic rocks start out as another kind of rock. Over very long times, one kind of rock can turn into another kind. The story never ends. It is a slow change, and it goes on and on.

Words to know

• mineral: natural solid
• igneous: cooled melted rock
• sedimentary: made from bits
• metamorphic: changed rock
• crystals: orderly mineral shapes

The short version

A rock is made of minerals. A mineral is a natural solid with a set chemical makeup and an orderly crystal structure. There are three main kinds of rock: igneous, sedimentary and metamorphic. The rock cycle is the never-ending change of rocks from one type into another over geologic time.

Quick facts

• Quartz is one of the most common minerals on Earth, with a Mohs hardness of 7.
• Diamond is the hardest natural substance known.
• Basalt, a dark volcanic rock, underlies most of the ocean floor.
• Sand grains are between 1/16 and 2 millimeters wide; bigger grains are called pebbles.