How do crystals grow inside rocks?

How it works

Everything is made of particles far too small to see. In a crystal, these particles line up like rows of children at a school assembly, repeating the same pattern over and over. As layer after layer stacks up, the outside naturally forms flat faces, and where the faces meet there are straight edges and sharp points.

Deep underground, melted rock called magma is a scorching liquid. When it cools very slowly, the particles have plenty of time to line up and form large crystals you can see, like the sparkly grains in granite. If lava cools quickly at the surface, only tiny crystals form, or none at all, making a glassy rock.

Crystals can grow from water too. Hot water underground carries dissolved minerals into cracks and hollows in rock. As the water cools or dries up, the minerals build up layer by layer into clusters of quartz or purple amethyst. Even the salt and sugar in your kitchen are tiny crystals.

Where to see it in New Zealand

The granite of Abel Tasman National Park formed as magma cooled slowly underground, and its crystals are big enough to see. Tūhua (Mayor Island), a volcanic island in the Bay of Plenty, is known for obsidian, a volcanic glass made when lava cooled too fast for crystals to grow. Māori used it to make sharp cutting tools.

Try it together

With an adult's help, stir spoonfuls of salt into warm tap water until no more will dissolve. Pour it into a dark shallow dish and leave it on a windowsill. When the water dries up after a few days, look for tiny cube-shaped crystals with a magnifying glass.