Diffraction of light by opals

Gem opals were examined in an optical diffractometer and found to give several types of diffraction pattern which are interpreted by analogy with conventional three-dimensional X-ray theory to give the structure of the opal. The results show that the spherical silica particles in opals are arranged hexagonally in layers which are usually stacked randomly. In some specimens there are parallel domains of ordered packing, commonly in a f.c.c. sequence and sometimes in a h.c.p. sequence. Although the silica particles cannot be resolved by optical microscopy, bands parallel to the layers and fringes across the bands can be seen in images with the diffracted light.