Three ways to make a rock
Every rock is made in one of three ways. IGNEOUS rocks form when melted rock, called MAGMA underground and LAVA on the surface, cools and sets hard. How fast it cools decides how it looks: GRANITE cooled slowly deep underground, so its crystals had time to grow big enough to see, while BASALT cooled quickly at the surface and its crystals are tiny. OBSIDIAN cooled so fast that no crystals formed at all, leaving natural glass. PUMICE is lava so full of gas bubbles that it floats; after an underwater volcano erupted near New Zealand in 2012, a raft of floating pumice covered hundreds of square kilometres of sea.
Sedimentary rocks, such as SANDSTONE, are built up from layers of SEDIMENT, sand, mud and pieces of shell, pressed and cemented together over millions of years. LIMESTONE is mostly made from the shells and skeletons of sea creatures, and it is where most fossils are found, as our seashells page explains. Metamorphic rocks are rocks changed by heat and pressure without melting. Limestone becomes MARBLE, and mudstone becomes SLATE, which splits into thin sheets perfect for roofs.
Where gemstones come from
A GEMSTONE is a MINERAL that grew as a clear, coloured CRYSTAL and is rare and hard enough to be worth cutting. Many grow deep underground, where hot fluids seep through cracks in rock and slowly deposit crystals. Colour often comes from a tiny trace of another element: a ruby and a sapphire are the same mineral, and a little chromium makes the ruby red.
A DIAMOND forms about 150 kilometres below the surface, where heat and pressure squeeze carbon into its hardest form, and it reaches us only when rare, deep volcanic eruptions carry it up. The largest gem diamond ever found, the Cullinan, was dug up in South Africa in 1905. It weighed over 600 grams, and the biggest stone cut from it is set in the British royal sceptre. A GEODE is a plain-looking round rock that, cracked open, is hollow and lined with crystals, often QUARTZ, one of the most common minerals in the Earth's crust.
Scratch tests and the oldest crystals
In 1812 the German geologist Friedrich Mohs came up with a scale of hardness from one to ten, based on which minerals can scratch which. Talc, soft enough to scratch with a fingernail, is one, and diamond, which scratches everything, is ten. Geologists still carry a steel nail and a piece of glass to test rocks in the field. Hardness is not the same as toughness, though: a diamond cannot be scratched by anything else, but a sharp blow in the right direction can split it, which is how diamond cutters shape a stone.
Some tiny crystals of a mineral called zircon, found in the Jack Hills of Western Australia, are about 4.4 billion years old, nearly as old as the Earth itself. At the other end of the story, EROSION by wind, water and ice wears down BEDROCK into a BOULDER and then into sand, and a FOSSIL may lie waiting in the layers. Our volcanoes page covers where fresh rock is made.