Natural Stones

What is this stone?

Narrow down the stone in your hand with a few observations and simple tests you can do at home. Skip any question you cannot answer. The result is a guess; for a definite identification ask a geologist, a geological survey or a museum.

Safety: do not taste stones or breathe their dust. Protect your eyes and hands in the vinegar test. Do not scratch a stone that might be valuable. How to do the tests →

What does it look like?
How does it shine?
What colour is it?
Scratch test

Try to scratch an edge with your fingernail, then with a steel knife; then try to scratch a piece of glass with the stone.

Streak colour

Rub the stone on unglazed porcelain (the back of a tile) and look at the colour of the powder.

How does it feel in your hand?
Does a magnet stick to it?
Does it fizz with a drop of vinegar?

As you answer, the most likely stones line up here.

Everyday name

Gravel and river pebbles

Gravel is rock fragments over 2 millimetres, rounded by rivers and waves; a river stone is not a kind of rock but any hard stone worn smooth by water.

Everyday name

Sand

Sand is not a kind of stone but a size: grains between 0.06 and 2 millimetres, mostly quartz, sometimes shell fragments or black magnetite.

Everyday name

Soil

Soil is not a stone: it is sand, silt and clay from crumbled rock, mixed with rotted plants, water, air and living things.

Rock

Andesite

The rock of volcanic arcs, named after the Andes — and the pink-grey stone of Ankara Castle.

Mineral

Augite (pyroxene)

The most common pyroxene, which darkens basalt and gabbro: inconspicuous, yet one of the key building blocks of Earth’s crust.

Rock

Basalt

The rock of the ocean floors and the Moon’s dark plains — the most common volcanic rock on Earth.

Mineral

Biotite

The shiny black flakes in granite: a dark mica that peels into thin sheets and can glitter like gold as it weathers.

Rock

Breccia

A rock broken and glued back together: its sharp, angular pieces tell of landslides, faults, volcanoes and asteroid impacts.

Everyday name

Chalk

Natural chalk is a soft, white limestone made of the shells of microscopic sea creatures; classroom chalk, however, is often gypsum.

Rock

Chert and flint

The steel of the Stone Age: a hard silica rock that breaks into razor-sharp edges and throws sparks when struck with steel.

Mineral

Chrysotile (white asbestos)

White asbestos: its heat-resistant fibres were used everywhere for a century, but when inhaled they cause cancer; it is now banned in many countries.

Rock

Claystone

Clay turned to stone: a raw material for bricks, studied as a home for radioactive waste because water barely passes through it.

Nothing looks right? Most stones people find are ordinary rocks. Is my stone valuable? →