Two of the most important tasks of the gemmologist are determining the genuineness and the quality of a gemstone. He identifies an unknown stone by measuring its optical and physical constants; he determines the authenticity and origin of the stone and its possible treatments to improve the optical appearance. The inner characteristics of a stone reflect the chemical and physical conditions during its growth in its natural surroundings – or, in the case of synthetic materials, in the laboratory. Such characteristics may be colour zones or natural intergrowth of two or more individual crystals according to crystallographic laws, also known as twinning. Often, foreign substances are included in gemstone minerals. Such inclusions are differentiated according to type:
- Protogenetic inclusions: Minerals already present before the host crystal formed and enclosed these minerals.
- Syngenetic inclusions: These were formed at the same time as the host crystal, e.g. fractures that have been healed. A lot of small cavities were formed in the process, so-called negative crystals, which contain remains of the aqueous solution in which the crystal grew.
- Epigenetic inclusions: Inclusions originating after the formation of the host crystal. These are mostly natural substances within fissures or exsolution products in the host crystal.
It is not necessarily true that a good gemstone has to be a hundred per cent clean and that inclusions are “defects” that diminish its value. In the case of faceted diamonds, the clarity of the stone is indeed a key criterion judged by a standardised nomenclature. For all other gemstones and ornamental stones, however, inclusions do not reduce their value provided they do not impair the stone’s appearance or stability. In fact, they make the stone unique and accentuate its exclusiveness – they are, as it were, nature’s signature. Apart from their scientific significance, the unique aesthetics of inclusions can often only be seen through a microscope. In view of this hidden inner beauty, the term “defect” assumes a positive rather than a negative meaning.
The microscopic examination of gemstones places great demands not only on the gemmologist but also on the instrument. As it is mostly stones set in jewellery that are examined, possibilities of observing inner characteristics are often limited. In some cases, immersion in liquid with light refraction similar to that of the stone can help. Extreme differences in contrast against a dark background, for instance due to reflecting facets or inclusions with metallic lustre, are also a considerable challenge for the illumination.
As a rule, stereomicroscopes with the following illumination techniques are used for gemmological examinations:
- Diffuse brightfield: This illumination enables observation of low-contrast growth structures, colour zoning and fluid inclusions. If crossed polarisers are used, it is possible to identify birefringent mineral inclusions or lamellar twin plains.
- Darkfield: Darkfield illumination shows up extremely fine structures such as needle-shaped or hairline inclusions that are not visible in brightfield.
- Glass-fibre optical waveguides: They enable a targeted darkfield illumination, or are used with incident light for the examination of surface structures.
For images like the ones illustrated in Figures 1–6 the Swiss Gemmological Society uses a Leica stereomicroscope with a Planapo objective 1.0x, which provides an adequate free working distance for examining even large objects. Illumination for brightfield or darkfield is supplied by a cold light source by Leica Microsystems. In addition, two glass-fibre waveguides with an external light source are used. As many gemstones are optically anisotropic, i.e. birefringent materials, a polarisation filter (analyser) is generally used to eliminate image blurring due to birefringence.
The documentation of inner characteristics of gemstones with the aid of photomicrography dates back to the 19th century. The German mineralogist Ferdinand Zirkel mentions this technique in 1873 in his book “Die mikroskopische Beschaffenheit der Mineralien und Gesteine“. All the same, Zirkel regarded photomicrography with some scepticism. Compared with the technique of drawing, he was of the opinion that photography offered no possibility of highlighting important parts of the image or omitting insignificant detail. Despite this supposed inflexibility, photomicrography became an indispensable instrument in gemmology as in other sciences. Gemmologists such as the Swiss professor Eduard Gübelin, one of the founders of the Swiss Gemmological Society, and the American J. Koivula played a major role in the further development of inclusion photography with black-and-white and colour film. Their techniques are still used in today’s age of digital photography. High-quality optics, careful work and a lot of patience are the preconditions for good photomicrographs, as the special characteristics of inclusion microscopy pose a considerable challenge. In particular, strong contrast, limited field depth as well as unnoticed specks of dust or scratches on the surface of the stone often create problems. As with microscopic analysis, different illumination techniques and, most importantly, their combination, are crucial for the results of photography.
Digital photomicrography, which has gained general acceptance over the last few years, opens up new possibilities for inclusion photography. Using the technique of High-Dynamic-Range Imaging (HDRI), photos can be produced of objects whose dynamic range of luminance between light and dark areas exceeds the limited luminance range of the photo sensor of the camera. HDRI photos are generated in the computer from a series of bracketed exposures, so that the full contrast range is stored in one 32-bit image. However, this image cannot be reproduced either on conventional monitors or with printing techniques. To obtain a realistic image corresponding to the visual impression with distinct highlights and shadows, a second step, so-called tone mapping, is carried out by compressing the luminance range to produce an 8-bit image that can be reproduced with conventional media.
Computer-aided postprocessing of the digital images offers the opportunity to overcome certain restrictions imposed by the still limited photographic technique and to obtain a more realistic picture. However, this tempting opportunity must not lead to retouching or colour changes that deliberately falsify the information provided by the image. In this respect, Ferdinand Zirkel is still right today: The author should use his possibilities of influence – whether with a drawing pen or image processing software to help document reality in an understandable way.
As a general rule: Gemmological examination methods must not be destructive.
- Gemmological stereomicroscope
- Refractometer (determination of refraction index)
- Hydrostatic balance (determination of weight and specific gravity)
- Spectroscope (optical observation of absorptions in the spectrum of visible light)
- Polariscope: determination of optical anisotropy
- Dichroscope: determination of pleochroism (differences in colour depending on the vibration plane of the polarised light)
- UV lamp: observation of fluorescence
Analysis techniques in modern gemmological laboratories
- Spectral photometer, UV-VIS and