What are the three types of rock?

Igneous (cooled and crystallised from molten magma or lava, e.g. granite, basalt), sedimentary (compacted and cemented sediment, often layered and fossil-bearing, e.g. sandstone, limestone) and metamorphic (an existing rock changed in the solid state by heat and pressure, e.g. marble, slate, gneiss).

What is the difference between a rock and a mineral?

A mineral is a single, naturally occurring inorganic solid with a definite composition and an ordered atomic structure (e.g. quartz). A rock is an aggregate of one or more minerals grown or bonded together (e.g. granite = quartz + feldspar + mica).

How does the rock cycle work?

Magma cools to igneous rock; igneous rock weathers and erodes into sediment that compacts into sedimentary rock; burial under heat and pressure turns rock metamorphic; melting returns it to magma. Shortcuts run almost everywhere, driven by Earth’s internal heat and by the Sun and gravity at the surface.

What is an example of each rock type?

Igneous: granite and basalt. Sedimentary: sandstone, shale and limestone. Metamorphic: marble (from limestone), slate (from shale) and gneiss (from granite).

Rocks, minerals & the rock cycle

We call almost everything underfoot “rock”, as if it were one dull grey stuff. It isn't. Rock is built from minerals, minerals are atoms in a repeating pattern, and the same atoms are recycled endlessly through three different rock-making processes. The powerbook below builds it from the atom up — so by the end you can pick up any rock and read how it was made.

rock families

5,000+

known minerals

endless loop

Rocks & the Rock CycleSection 1 of 6

0 of 6 locked in

This book has 6 short sections. Read each one, answer the check, then use Next to move on — the bar above tracks how far you've come.

What a mineral really is

Pour a pinch of table salt onto dark paper and look closely. Every grain is a tiny cube — flat faces, square corners. Beach sand isn't. Why would salt grow itself into cubes?

Because salt is a mineral, and a mineral is not just "a hard bit" — it is atoms locked into a repeating three-dimensional pattern, a crystal lattice. In table salt, sodium and chlorine atoms alternate in a perfect cubic grid, and that hidden cubic order is what forces each whole grain to grow as a visible cube. The shape you can see is the atomic pattern you cannot.

Geologists tighten this into a definition: a mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered atomic structure. Quartz is the mineral SiO₂ — one silicon to two oxygen — repeated almost endlessly. There are over 5,000 known minerals, but only a few dozen are common, and a mere handful (the silicates, built on that silicon–oxygen unit) make up most of the rock around you.

So before we talk about rock at all, we start one level down. Rock is built from minerals — and a mineral is simply order made visible.

New words

mineral

a naturally formed solid with a fixed recipe and an ordered, repeating atomic pattern (e.g. quartz, salt)

crystal lattice

the repeating 3-D grid of atoms inside a mineral — the reason crystals have flat faces and regular shapes

🔒 Lock it in

Why does a grain of table salt form a neat cube?

Answer the check to continue

Quick reference & sources

The interactive powerbook above is the lesson — read it section by section with the Next button. What follows is a plain recap and the sources, for when you only need the facts.

The three rock families, at a glance

How each forms, an example or two, and the clue that gives it away.

Igneous — magma or lava cools and crystallises. Granite (slow, big crystals), basalt (fast, tiny crystals). Clue: interlocking crystals; grain size shows cooling speed.

Sedimentary — sediment is compacted and cemented. Sandstone, shale, limestone. Clue: flat layers (strata) and fossils.

Metamorphic — an existing rock is changed by heat and pressure, without melting. Marble (from limestone), slate (from shale), gneiss (from granite). Clue: banding (foliation), recrystallised, harder.

How to read a rock

Three quick questions place almost any rock. Are there interlocking crystals? Then it crystallised from a melt — igneous, and the grain size tells you how fast (big = slow and deep, small = fast at the surface). Are there layers or fossils? Then it was built from sediment — sedimentary. Is it banded or streaked, recrystallised but never melted? Then it was cooked and squeezed — metamorphic.

A fourth clue is hardness. Minerals are ranked on the Mohs scale, devised by Friedrich Mohs in 1812: from talc (1, scratched by a fingernail) up to diamond (10, the hardest natural material). Quartz sits at 7 — harder than steel — which is why quartz-rich rocks resist wear and quartz sand outlasts almost everything else on a beach.

For HSC Earth & Environmental Science students

This powerbook covers the “Rocks, minerals and the rock cycle” thread of Module 1: Earth's Resources: what defines a mineral, the properties used to identify minerals and rocks, the three rock types and how each forms, and the rock cycle and the energy that drives it. It connects directly back to What's inside the Earth — the internal heat that powers melting and metamorphism — and forward into how those resources are found and used.

It's one book in a course we're building thread by thread. See the full map on the Earth & Environmental Science hub.

Sources & notes

Definition of a mineral (naturally occurring, inorganic, definite composition, ordered structure) follows the International Mineralogical Association (IMA); over 5,000 distinct mineral species are recognised.
Silicate minerals make up roughly 90% of the Earth's crust — standard petrology.
The Mohs hardness scale (talc 1 to diamond 10) was devised by the German mineralogist Friedrich Mohs in 1812.
The three-fold rock classification (igneous, sedimentary, metamorphic) and the rock cycle are standard introductory geology; transformations cited (limestone→marble, shale→slate→schist, granite→gneiss) are textbook examples.
Course framing follows the NSW NESA Stage 6 Earth and Environmental Science Syllabus (2017). Explanations here are original and not a reproduction of the syllabus; this site is not affiliated with NESA.

Keep exploring

The grain of quartz in your hand has been lava, sand, and stone before — and will be again. Rock is not a thing; it is a stage in a loop that never stops.