Rocks

Naturally occurring aggregates of minerals, and what they are quarried or mined for.

• Banded iron formation

  Alternating layers of iron oxide and chert laid down in Precambrian seas — the source of nearly all the iron ore mined on Earth, and a record of the atmosphere acquiring oxygen.

• Basalt

  The dark fine-grained volcanic rock that forms the ocean floor — by area the most abundant rock at the Earth's surface.

• Bauxite

  A weathered residual rock rich in aluminium hydroxides — the world's only significant aluminium ore.

• Carbonatite

  An igneous rock made of carbonate minerals rather than silicates — geologically improbable, and the source of most of the world's rare earths and much of its niobium.

• Chalk

  A limestone made almost entirely of microscopic algal plates — soft, porous, white, and not what a blackboard stick is made of.

• Coal

  Compressed plant matter that became rock — the fuel the industrial revolution ran on, and still the reducing agent that most of the world's steel is made with.

• Evaporite

  Rock precipitated where a body of salt water dried up — the source of table salt, of potash fertiliser, of gypsum for plaster, and of borates.

• Flint

  Silica so fine-grained it breaks like glass in a predictable direction — the material of the first tools, and of the first fires struck deliberately.

• Gabbro

  Basalt's coarse-grained equivalent, cooled slowly at depth — and, where it settled into layers, the source of the world's platinum, most of its chromium and much of its nickel.

• Gneiss

  High-grade metamorphic rock banded light and dark by its own minerals separating — and the commonest rock in the ancient cores of the continents.

• Granite

  A coarse-grained igneous rock of quartz and feldspar, and the archetypal building stone.

• Greisen

  The top of a granite chemically rebuilt by its own escaping vapours into quartz and mica — the classic host of tin, and of the tungsten and molybdenum that accompany it.

• Kimberlite

  A volatile-rich volcanic rock erupted from mantle depths at extreme speed — the only rock that delivers diamond to the surface intact.

• Limestone

  A sedimentary rock composed mainly of calcite, and the feedstock for lime and cement.

• Marble

  Metamorphosed limestone — recrystallised calcite, and the sculptor's stone.

• Obsidian

  Volcanic glass, black and sharper than any steel — the other tool stone, and the one that can be dated and sourced precisely enough to map ancient trade.

• Pegmatite

  The very coarse last fraction of a granite to crystallise — crystals measured in metres, and the host of lithium, beryllium, caesium, tantalum and the rare earths.

• Peridotite

  The rock the Earth's mantle is made of — olivine and pyroxene, rarely seen at the surface, and the source of chromium and much of the world's magnesium.

• Sandstone

  Cemented sand — the most-used building stone in much of the world, and the rock that holds most groundwater and oil.

• Schist

  Metamorphic rock rebuilt with all its mica lying the same way, so it splits into glittering sheets — the host of industrial talc, graphite and emery.

• Serpentinite

  Mantle rock that met water and changed — slippery, green, and the source of most industrial talc as well as of asbestos.

• Shale

  Compacted mud, and the commonest sedimentary rock on Earth — the source rock for nearly all the world's oil and gas, and the seal that traps it.

• Skarn

  Limestone recrystallised and chemically rebuilt where an intrusion cut through it — the world's main source of tungsten, and a significant one for copper, iron, zinc and molybdenum.

• Slate

  Metamorphosed mud that splits into flat sheets — the roofing material that made dense cities weatherproof.