Many people know that graphite and diamond are both pure carbon – the only difference is that the carbon atoms are arranged differently in the two types of crystals. In graphite the carbon atoms are arranged in sheets, which can slide past each other easily allowing graphite to be used in pencils, while in diamond the carbon atoms are arranged in a similar way to ice. Or rather like the water molecules are in the most common form of ice, while there are two forms of carbon there are 15 known different forms of ice.

Graphite and diamond are, I think, on the A level Chemistry syllabus and are referred to as allotropes. But the general name for different crystals made from the same substance, like the 15 different types of ice, is polymorph. The word is from the Greek poly meaning many and morph, meaning form or shape. Ice forms crystals of 15 different shapes.

The fifteen different ices is impressive but most of these only appear at extremely high pressures. Other substances can form a whole range of different pretty crystals in a lab at atmospheric pressure. The champion here is called ROY, as it forms Red, Orange and Yellow crystals. A scientist in the USA has written a paper on this and in the abstract he has images of several of these beautiful crystals. Not only are they all different colours but the way the crystals grow means some are needle-shaped, some like lozenges, etc, all made from exactly the same molecules.