Most rocks look plain, but each one is a record of heat, pressure, water or time. To read it, geologists use a few exact terms. A mineral is a natural solid with a set chemical makeup and an orderly crystal structure. A rock is a mix of one or more minerals. A mineral's hardness is its resistance to scratching, found by scratching it against a substance of known hardness on the Mohs scale. Luster is how light reflects from its surface: metallic, glassy, dull or earthy. Cleavage is a mineral's tendency to break along weak planes. Each of these is a clue, and none of them is enough alone.

Diamond is rated 10 on the Mohs scale and is about four times harder than corundum, which is 9. So the numbers are rankings, not equal steps. Think of a race: first and second place do not tell you how far apart the runners were. A small gap in the numbers can hide a big gap in hardness. Diamond is the hardest natural substance known, yet its hardness is directional. It is hardest parallel to its octahedral planes and softest parallel to its cubic planes. Industrial diamonds are used mostly for cutting, grinding, drilling and polishing, where hardness and heat conduction matter more than looks.

Same carbon, two very different minerals. Diamond and graphite are both pure carbon, yet graphite is extremely soft. Diamonds form in Earth's mantle, about 100 miles (160 kilometers) down, where the heat and pressure are high. Most diamonds found have been carried up by deep-source volcanic eruptions. Graphite forms when carbon is heated and squeezed in Earth's crust or upper mantle. That takes roughly 75,000 pounds per square inch and 750 degrees Celsius (about 1,380 degrees Fahrenheit). Many people think diamonds form from coal. Geologists say that is not true.

Look around a room and you will find minerals at work in more places than you might guess. Minerals do jobs far from the museum case. Gems are mostly very rare minerals, unlike the slightly more than a dozen common minerals that make up most of what we see. Quartz is used in the tiny crystal oscillators inside watches, clocks, radios, computers, cell phones and GPS equipment. Feldspar is one of the most common minerals on the planet, and it is mostly used for glassmaking and ceramics. Potassium feldspar is typically pink-salmon to white, and it has a Mohs hardness of 6.

Igneous rocks show how fast they cooled. Fast cooling leaves mineral crystals little time to grow, so extrusive rocks are fine-grained or glassy; trapped gas bubbles can leave a bubbly texture. Obsidian breaks into curved surfaces and very sharp edges, which may be why people first used it as a cutting tool. Basalt underlies more of Earth's surface than any other rock type, including most of the ocean basins. It is also abundant on the Moon, where much of the surface is underlain by basaltic lava flows called lunar maria. Rock from fire covers much of two worlds, and it is easy to see why. Each of these rocks is a record of fast cooling.

Slow cooling tells the opposite story. Granite is a coarse-grained, light-colored igneous rock that forms by the slow crystallization of magma below Earth's surface. Half Dome in Yosemite stands nearly 8,800 feet (2,682 meters) above sea level and is the remains of a magma chamber that cooled slowly thousands of feet underground. Introductory geology textbooks say granite is the most abundant rock in the continental crust, and it shows up in the cores of many mountain ranges. The size of the crystals is the clue. It tells a geologist how long the rock took to cool. Big crystals mean a long wait.

Rock can be built in steps from loose bits. Weathering breaks rock into fragments. Wind, water, ice or living things carry them away. Once enough sediment settles, the lowest layers are compacted so tightly that they form solid rock. Lithification is how clay, sand and other sediments on the bottom of the ocean or other bodies of water are slowly compacted into rock by the weight of the sediments above. Clastic sedimentary rocks are named by grain size. Shale is mostly clay, sandstone is sand-sized, and conglomerate is pebbles surrounded by sand or mud.

Limestone can form from living things, such as shell and coral debris, or by chemical precipitation from lake or ocean water. Chalk is a soft limestone made from the shells of microscopic sea organisms. In caves, stalactites and stalagmites form when water picks up calcium and carbonate from bedrock, then evaporates and leaves calcium carbonate on the ceiling or floor. Coal forms from plant debris that piles up in oxygen-poor swamp water, where lack of oxygen keeps the plant debris from decaying. So coal begins as a swamp, and it is a rock with a plant for a parent.

Metamorphism creates new minerals, either by rearranging a rock's mineral components or by reactions with fluids that enter the rock. Foliation is the striped look that forms when pressure squeezes flat minerals so they line up; limestone does not align no matter how much pressure is applied. Regional metamorphism can change shale into slate, then phyllite, then schist, and finally gneiss. Contact metamorphism happens when hot igneous rock pushes into older rock and bakes it, changing its minerals without added pressure. Heat alone can change a rock, and so can squeezing, and either one leaves new minerals behind. Rocks are not fixed forever, and the heat and pressure leave their marks.

How do we know all this? Thin sections, slices of rock or mineral only 0.03 millimeters thick mounted on glass slides, are how most optical mineralogy is done today. Radiometric dating measures the age of rocks from the decay of long-lived radioactive isotopes of elements found naturally in rocks and minerals. Earth's oldest rocks have been recycled and destroyed by plate tectonics, so scientists cannot find the exact age of the Earth directly from Earth rocks. The rock cycle keeps erasing its own oldest pages, so scientists read other clues instead. So every rock that is left is a clue worth a close look, and each clue adds to the story.

Words to know

• luster: surface shine
• cleavage: breaking pattern
• lithification: turning to rock
• foliation: striped layering
• radiometric: decay-based
• extrusive: surface-cooled