Rockhounding wiki: rocks, minerals and geology

How the rocks under your boots formed, explained in plain language, with links to the places where you can see each one.

What are the three types of rock?

Rocks form in three ways. Igneous rock cools from molten magma, like granite and obsidian. Sedimentary rock settles in layers and hardens, like sandstone and limestone, and holds most fossils. Metamorphic rock is any existing rock changed by heat, pressure or hot fluids without melting, like marble and slate.

Browse the wiki

Rocks, minerals and gemstonesRocks, metals, minerals, crystals and gemstones, one entry each.GeologyEarth materials, structure, processes, mapping and time.MineralogyWhat a mineral is and how minerals are classified.PetrologyIgneous, sedimentary and metamorphic rock.CrystallographyThe seven crystal systems, symmetry and how crystals grow.GemologyCut, colour, clarity and the tests for fakes.LapidaryCutting, polishing, treatments and jewellery making.BirthstonesThe stone for each month and where to find it.

Start with these rocks

IgneousrhyoliteLight-coloured volcanic rock, the host of thundereggs and topaz.IgneoustuffRock made of volcanic ash, often soft enough to carve.SedimentarysandstoneSand grains cemented together, the red cliffs of the Southwest.MetamorphicgneissBanded rock formed under great heat and pressure.IgneousdioriteSpeckled black and white rock that cools slowly underground.MetamorphicschistFlaky, glittery rock rich in mica; it holds garnets.

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Photo credits, Wikimedia Commons: Granite by James St. John, CC BY 2.0; Sandstone by Darla Sondrol, CC0; Gneiss by James St. John, CC BY 2.0.