The dictionary definition of crystal is clinical, referring to “forms assumed by substances with a definite internal structure and external shape of symmetrically arranged plane surfaces”. But the idea that crystals are cold and static has persisted since the time of ancient Greece — the term crystallos ('ice' or 'quartz') evolved from cryos ('cold') and halas ('salt'). As recently as the mid-1900s, the Nobel prizewinner Leopold Ruzicka dismissed crystals as “chemical cemeteries”. Crystals were dead and so no chemistry could take place within them.

Physicists have taken a phenomenological approach. According to the International Union of Crystallography, a crystal is any solid that gives a discrete X-ray diffraction diagram. A crystal is thus defined not by what it is but by how it appears. But when an entity is defined by an observed property, its very existence depends on the method of observation. Crystals were first observed by eye, then by microscope, then by X-rays. What will be next?

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