Replica and transmission electron microscopy have been used to study the microstructure of precious opal. This mineral is an amorphous silica which is often prized as a gemstone because of its vivid display of iridescent colors when viewed in visible light. For the present study, specimens were examined in fractured, polished, and etched conditions.As shown in the figures, precious opal consists of many tiny silica spheres, of essentially uniform diameter, that have been deposited in an orderly array before silica cementation. Regularly spaced interstitial voids resulting from the ordered packing serve as a three dimensional diffraction grating for visible light, thus giving rise to the characteristic play of colors. The separation of the voids is controlled by sphere diameter, and determines the maximum wavelength of visible light diffracted. Thus specimens consisting of larger spheres diffract higher order colors (longer wavelengths) than specimens consisting of smaller spheres. For example, the specimen shown in Fig. 1 contained spheres of approximately 2700 Å diameter and diffracted all visible colors up through red, while the specimen shown in Fig. 2 contained spheres of approximately 2000 Å diameter and diffracted only blues and greens.
Self-assembly in the growth of precious opal
