Born of fire
Igneous
Molten rock cools and locks into crystals. Slow cooling deep underground grows large ones; a fast chill at the surface leaves barely any at all.
- Granite
- Basalt
- Obsidian
- Pumice
A field guide for young geologists
Every rock is a record of something that happened.
Fire, water, and pressure each leave a signature you can learn to read. Start with the three families, then test yourself — no hammer required.
Sandstone strata in a sea cliff — youngest layer on top.
01 — Classification
Three families, three origin stories
Every rock on Earth belongs to one of these, and any of them can become another given enough time. Geologists call that loop the rock cycle.
Built in layers
Sedimentary
Sand, silt, and the shells of sea creatures settle, stack, and compact. Each band is a slice of time — which is why canyon walls read like pages.
- Sandstone
- Limestone
- Shale
Changed under pressure
Metamorphic
Bury a rock deep enough and heat and pressure rearrange it without ever melting it. Limestone recrystallises into marble; shale hardens into slate.
- Marble
- Slate
- Gneiss
- Schist
02 — Specimen of the day
Look closer
Igneous
Obsidian
Volcanic glass. Lava cooled so quickly that crystals never had time to form, leaving a smooth, dark surface that fractures into edges sharper than a surgical blade — which is exactly what ancient toolmakers wanted.
- Hardness
- 5 – 5.5
- Found in
- Lava flows
03 — Field facts
Six things worth knowing
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About 4 billion years
The age of the oldest rocks yet found on Earth — nearly as old as the planet.
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Pumice floats
It traps so much volcanic gas that rafts of it can drift across entire oceans.
-
100 miles down
Roughly where diamonds crystallise, under heat and pressure hard to imagine.
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Limestone was alive
Much of it is built from the shells and skeletons of ancient sea creatures.
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Talc 1, diamond 10
The Mohs scale ranks minerals by what can scratch what.
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Geodes hide inside
A dull grey lump outside; a cavity lined with crystals once it's opened.
04 — Check yourself
Four questions
05 — In the field
Starting a collection
Keep them separated
An egg carton or a partitioned box stops softer specimens from being scratched.
Record the location
Where a rock was found is half its story. Note the date and place before you forget.
Test the hardness
A fingernail, a copper coin, a steel nail, a piece of glass — in that order, hardest last.
Collect responsibly
Ask permission on private land, and never chip at protected outcrops or park formations.