The structure and optical behaviour of chalcedony

Summary

Common chalcedony and agate, when examined by methods of investigation similar to that employed in the case of iridescent agate, the results of which have already been reported in theseProceedings, exhibit analogous features. The transmission and diffusion of light, their polarisation characters and the influence of the thickness of the medium on these features are discussed from a theoretical standpoint. The results are borne out by observations made with several specimens of agate and chalcedony. The lamellar structure which is often exhibited by chalcedony is found to arise from the orientation of thec-axes of the crystallites of quartz in the fibres along the banding planes which run transversely to the former. X-ray patterns of chalcedony in such regions reveal that the crystallites of quartz are orientated with theira-axes common along the fibre length.