Examination methods of the light – optical microscopy, especially as polarizing microscopy, are essential in gemmology. The construction principle of a microscope with objective, tube and ocular remains the same at all constructive solutions. As gemstone microscope the tube is mostly horizontal, at all other usual microscopes it is vertically arranged. Today the viewing is arranged nearly only through a binocular inclined tube.

The lighting can be made with reflected light or transmitted light. In the first case the light of one or several lamps or daylight comes sideways directed from above onto the object, and the reflected amount coming into the objective give a picture of the object. The lighting is made in reflected light – dark field, i.e. outside the objective – aperture. Therefore we call the lighting through the objective, caused by sideways incidented into the tube and deflected there light as reflected light – bright field. This arrangement is used especially for ore- and metallurgical microscopy. In reflected light contours and colours can be observed very good.

For transparent objects, and that are gem stones in general, most effective is used the transmitted light microscopy, namely in transmitted light – bright field, where the lighting aperture is within the observation aperture of the objective. With the help of the polarizing – optical attachment, a polarizing filter in front of the object as polarizer and a second polarizing filter behind the object as analyser as well as different compensators, the crystal – optical properties can be detected excellently and measured very precise. Such data are necessary for an ensure gemstone detection. Furthermore the transmitted light microscopy makes possible a lot of additional assessments. Inclusions or defects can give an answer on the question if it is natural or synthetic, if it has been treated and where is its origin. Such high claims require qualitative sophisticated microscopic equipment and a lot of experience.

If you work only with diamonds, the gem stone diagnostics does not play a role, a more detailed utilization of microscopy is necessary. Although the diamond is according to its cubic point group optical isotropic, nearly all diamonds show under the microscope at crossed polarizers an optical anisotropy. The stress distribution in the crystal, caused by distribution inhomogeneities, inclusions, crystal structure defects, deformations or growth anomalies, create different and often very characteristic patterns of interference colours and levels of the pitch of grid differences. From this important indications concerning diamond type and inner state can be deducted.

In normal transmitted light – bright field in the diamonds microscopically can be seen manifold additional topographic features. Characteristic for many rough natural diamonds are etch pits on the surfaces, namely so called trigons on the octahedral faces or rectangles on the rhombododecahedral faces, growth- or solution steps and banded structures with different causes.

Inclusions can be seen often already topographically, as for example the relative frequent, mostly epigenetic graphite. Others can be detected with optical features. Important is the microscopic determination of microcracks and other large defects. To them belong for example the platelets, strip-like things with longitudinal direction parallel to the normal of the rhombododecahedral faces in planes parallel the cube faces, which sometimes in their largest formation with 1 x 20 μm can be shown. They indicate higher nitrogen contents and deformations.

Further topographic details can be imaged by use of certain types of lighting, as the transmitted light – dark field, often variable combined with the transmitted light – bright field and/or the reflected light – dark field, the two-beam- or multiple-beam interferometry and the phase contrast. A kind of luminescence microscopy is the UV-topography, where at excitation with UV-light the luminescence distribution is detected microscopically. The schlieren microscopy, the lighting is made in transmitted light in the narrow boundary area of bright- and dark field, is for diamonds of type Ia very important. At this in parallel schlieren or Stripes can be seen a changing concentration of nitrogen. This effect bases on the low differences of the refraction of light between schlieren with higher nitrogen contents (higher refractive index) and lower nitrogen contents (<n). Length and width (last-named approx. <1 up to 5 μm) can clearly vary (also within one diamond). Higher nitrogen contents are indicated often by a good visibility.