Insect and quartz optical phenomena. 19th-century illustration of quartz and insects as examples of optical phenomena. The central illustration shows two layers of overlapping quartz, with the resulting display of colours due to a phenomenon called birefringence, a combination of polarisation and refraction. The colours of some insects, such as beetles, are due to a phenomenon called iridescence, due mostly to interference and diffraction of the light as it hits the layers in the insect casings. This illustration is from 'Physique Populaire' (Emile Desbeaux, 1891).