Opal

Rome already knew opal was unlike anything else. Pliny the Elder, writing in the first century, described a stone that carried the fire of other gems at once, the purple of amethyst, the sea-green of emerald, shining together in a union he found scarcely believable. What Rome could not know was why.

The answer waited nearly nineteen hundred years, until 1964, when J. V. Sanders and his colleagues at Australia’s CSIRO placed precious opal under an electron microscope and published what they saw in Nature: spheres of silica, each a few hundred nanometres across, stacked in ordered rows. The stone is a natural diffraction grating, its colour is not pigment but pure geometry, light divided by architecture too small to see.

It is fitting that the answer was Australian. Since a fourteen-year-old boy found opal at Coober Pedy in 1915, Australia’s fields have supplied roughly ninety-five per cent of the world’s precious opal. Every HAS opal dial holds that geometry: an ancient wonder, explained at last, and not diminished by the explanation.