Anisotropic opaque minerals viewed in reflected light microscopy show two sets of colours: the colours seen in plane polarized light which change as the section is rotated on the microscope stage, and the colours seen between crossed polars which change as the analyser is uncrossed. These latter colours are known variously as polarization colours or anisotropic rotation tints, but are here referred to as anisotropy colours. They are commonly a diagnostic aid to correct mineral identification. All these colours occur as a consequence of the dispersion of the relative permittivity (dielectric) tensor—the variation in the values of the tensor with wavelength of incident light and in low symmetry crystals, the variation in the directions of the principal axes of the tensor with wavelength.

In this paper, it is shown that the colour seen in plane polarized light and the anisotropy colours can be predicted for any orientation of section, at any stage angle, and for any degree of uncrossing of the analyser by calculations based on the dielectric tensor values, and these predicted colours compare favourably with the observed values. Three minerals are studied in this paper as examples: stannite, covelline, and bournonite.

From the late second century AD to the end of the fourth century AD, black, shiny materials were popular in Britain and the Rhineland for the production of jewellery. In the past, in the absence of accurate, detailed analysis, all these products, irrespective of composition, have invariably been described as being made of ‘jet’. A project at the University of Newcastle upon Tyne, UK, has used the non-destructive technique of reflected light microscopy to identify the various black materials used by the Roman craftsmen. Long used for petrographic studies of coals and fossil fuels, this technique has proved very suitable for the analysis of small artefacts. The project initially focused on black objects from the area of Hadrian's Wall but was extended to look at objects from Rhineland sites as well as finds from other parts of Britain and the Continent. The project has revealed that a wide range of ‘jets’ and shales and varieties of coal from a diversity of geographical sources were used, and that the Rhineland objects were not carved from Whitby jet but from an unidentified source.