Rocks and Minerals

The whole solid Earth is built from rocks, which themselves are made up of minerals. There are over 5,000 known minerals (each with its own particular crystal structure and chemistry), and they combine in many ways to make the rocks we see all around us. Rocks come in three big types based on how they formed: igneous (from cooled magma), sedimentary (from compressed layers), and metamorphic (changed by heat and pressure). All three are constantly being recycled in a giant slow loop called the rock cycle.

• Three rock typesIgneous, sedimentary, metamorphic
• Known mineralsOver 5,000And a handful of new ones found each year
• Hardest natural mineralDiamondMohs scale 10
• Oldest known rocksApprox. 4 billion yearsAcasta Gneiss, Canada
• Most common mineralFeldsparAbout 60% of Earth's crust
• Most famous gemstoneDiamondPure crystal carbon

The three types of rock

• Igneous: formed when hot molten rock (magma) cools and hardens. Examples: granite, basalt, obsidian, pumice. The word comes from the Latin ignis, meaning fire.
• Sedimentary: formed from compressed layers of sand, mud, plant remains, sea shells and other small bits. Examples: sandstone, limestone, chalk, coal. Most fossils are found in sedimentary rock.
• Metamorphic: formed when older rocks are changed by intense heat or pressure deep in the Earth. Examples: marble (changed from limestone), slate (from shale), gneiss (from granite). The word means "changed in form".

The rock cycle

Rocks are not made once and left alone. They are constantly being changed, broken down, melted and re-formed in a giant slow loop called the rock cycle. Wind and water break rocks into tiny bits that wash into rivers and oceans, where they pile up as sediment and slowly become sedimentary rock. Tectonic forces push some rocks deep underground, where heat and pressure turn them into metamorphic rock. If they go deep enough, they melt into magma, which can rise back up, cool, and become new igneous rock. The whole cycle can take tens of millions of years.

Fact A diamond is just pure carbon arranged in a special crystal structure formed under enormous pressure deep below the Earth's surface. Graphite (the soft black stuff in pencils) is also pure carbon, but with the atoms arranged differently. So a pencil lead and a diamond are made of exactly the same atoms, just in completely different patterns.

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