optics being indicated by 1 whereas the dark ground lighting proper is formed e g by an annular lamp 2 which is disposed behind a screen 3 relative to the microscope optics 1 whereas a black background 5 is provided behind the precious stone 4 being examined still relative to the microscope optics 1 fig2 shows the very simple ring lighting according to the invention said lighting being mainly formed for the examination of a precious stone 4 by a separate source of light 5 a fiberglass light conductor 6 known per se and a reflector 7 as it is known the irradiated by the source of light 5 is collected by a bundle of glass fibres 8 the fibres of said bundle being suitably distributed in an annular end piece 9 in order to transmit an annulus of light to the upper surface of said end piece 9 the light irradiated through the end piece 9 is then collected by the reflector 7 which is therefor provided with an internal conical surface 10 so that by using a light source 5 having a high light intensity a large amount of light is thereby projected onto the precious stone being examined the light rays reflected by the reflector 7 being maintained as nearly horizontal as possible relative to the vertical observation direction of the precious stone 4 so that by locating the source of light 5 at a suitable distance from the observation location the heat evolved from said source has no influence on the observation in order to maintain the reflections on the polished facets of the precious stone at the.