Minerals & crystals

Pick up any stone and look at it closely. Often you can see small glittering or differently coloured grains. These are minerals, the building blocks of rocks. A rock is usually a mixture of several minerals, just as a cake is made from flour, sugar and eggs.

What is a mineral?

A mineral is a natural, solid material in which the atoms are arranged in a fixed, always identical order. It is exactly this order that separates a mineral from a random lump. Salt, quartz and the graphite in your pencil are minerals. There are thousands of different ones, and each has its own properties such as colour, shine and hardness.

Crystals grow into shapes

When a mineral can grow undisturbed, the atoms arrange into a regular pattern. On the outside this shows up as a crystal with flat faces and sharp edges. Table salt forms tiny cubes, quartz often forms six sided columns with a point. The shape is no accident, it mirrors the hidden order of the atoms inside.

How hard is hard? The Mohs scale

To compare minerals, people use the Mohs scale with 10 levels. Whatever ranks higher scratches everything below it. Talc at level 1 is so soft that you can scratch it with your fingernail. Quartz sits at level 7, diamond at the very top on level 10 and is the hardest natural mineral. A harder mineral always leaves a scratch on a softer one, never the other way round. The scale only tells you the order, though, not how big the gaps are: from level 9 to level 10 the jump in real hardness is far bigger than from level 1 to level 2, even though both are just one step.