Crystals

Take a really close look at a grain of salt, best with a magnifying glass: it is a tiny cube with smooth faces. Grains of sugar glitter, snowflakes have six points, a diamond sparkles in the light. All of these are crystals, and their shapes are no accident. Here you'll see why that is.

Order in the particle picture

In a crystal the particles do not sit around in a wild jumble but in a strict, endlessly repeating order. This regular pattern is called a lattice. Picture it like a perfectly stacked tower of identical building blocks: row by row, layer by layer, always at the same spacing. Because the pattern is so regular on a small scale, the whole crystal also gets smooth faces and straight edges on the outside.

How crystals grow

Many crystals grow from a saturated solution. Saturated means: at this temperature no more substance can dissolve in the water. If some water now slowly evaporates, it gets too crowded for the dissolved particles, and they attach to the lattice one after another. The slower this happens, the bigger and finer the crystals become. Sugar crystals on a thread in a glass grow this way over days, and in nature some gemstones take many thousands of years. It also works without any water: when molten rock cools down slowly, the particles line up into crystals just the same.

Order against disorder

The difference between a crystal and a liquid like water is easy to see in the particle picture: in the crystal the particles are firmly ordered, in water they move freely and in disorder. That is why a grain of salt has a fixed shape but a drop of water does not. A snowflake is made of ice: in the cloud, water vapour turns straight into ice without ever being liquid, and because the ice lattice is six-sided, every flake gets six points. When it melts, the order dissolves again.