The structure of opal and the origin of its iridescence

Summary

A study of the optical behaviour of iridescent opal indicates very clearly that the silica present in the material has a regularly stratified structure in which the alternate layers differ in refractive index, such difference being small but the same throughout the stratifications. A critical examination of the X-ray diffraction patterns of cryptocrystalline hyalites exhibiting optical phenomena identical with of analogous to those of precious opal confirms this finding and enables the two species of silica present in association with each other and giving rise to these phenomena to be identified respectively with high and low cristobalite. The different optical effects exhibited by a stratified medium when the spacing of the stratifications and the extent of the domains in which they are present are varied have been observed with different specimens of hyalite and are illustrated in the paper by a series of photographs.