Abstract
THE precious variety of opal, a hydrous colloidal form of silica, is iridescent. Explanations have been offered of the origin of the observed play of colours1,2, some including the most recent optical diffraction evidence3. Electron microscope studies were thought to indicate that the colours may originate from spherical silica particles arranged hexagonally in layers of predominantly random stacking. The studies also contributed measurements of particle diameter and approximate shape4–6. Replication difficulties of sample preparation, however, inhibited these investigations, particularly affecting the search for three-dimensional information.
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