Exclusive Aesthetics of Nature

Inclusions in Gemstones

December 15, 2009

Gemstones have fascinated people for thousands of years. Rulers and kings used to demonstrate their power and wealth with jewel-studded insignia. Although fine jewellery is still a status symbol of the rich, we now tend to treasure these wonders of nature more for our own pleasure in beauty and harmony. The place where aesthetics of the mineral world and science meet is the domain of gemmologists. In modern gemmological laboratories, microscopic examination of the interior and surface characteristics of a gemstone is still the mainstay for assessing quality criteria.

Different types of inclusions

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.
Fig. 1: Inclusions probably of melanter-ite (hydrous Fe sulphate) in rock crystal (quartz), found at Minas Gerais, Brazil. It is a pseudomorphis after the iron sulphide marcasite, which is also found as inclu-sions in quartz in the same deposit. Trans­mitted light, crossed polarisers.

Microscope reveals hidden beauty

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.

Fig. 2: Inclusion of a tourmaline crystal in aquamarine (variety of beryl), found in Northern Pakistan. The interference colours of the tourmaline needles are due to the polarised light. Width of image: approx. 1.5 mm, transmitted light, singly polarised light.

Illumination techniques

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.

Fig. 3: Fluid inclusions and growth zoning in aquamarine (variety of beryl), found at Mina Padre Paraiso, Minas Gerais, Brazil. During the growth of a crystal in an aqueous solution, stress can cause the formation of fractures that heal as the crystal continues to grow. In the former fissure, remains of the original aqueous solution are sealed in cavities, the so-called negative crystals. These fluid inclusions are located in the plane of the former crack. Their contents separate in different phases as the stone cools down after crystal growth. In the illustrated sample, all inclusions contain a gas bubble (CO2), liquid CO2 and a small amount of water. Width of image: approx. 3 mm, transmitted light, crossed polarisers, first-order red compensator.

Digital photomicrography of inclusions

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.

Fig. 4: Rock crystal (quartz) and rutile needles enclosed in rock crystal (quartz), from Minas Gerais, Brazil. Because of the light metallic lustre of the rutile needles (titanium oxide, TiO2), the trade name of this quartz variety is “platinum quartz”. The picture shows a first generation rock crystal inclusion that was enclosed by a second generation of quartz. Although the inclusion and the host material have the same refractive index, the enclosed quartz is easily recognisable due to a thin film of air at the interfaces. Width of image: approx. 6 mm, transmitted light, crossed polarisers, first-order red compensator.

Processing without falsifying

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.

Fig. 5: Fluid inclusions and inclusions of rutile needles and small siderite crystals (iron carbonate) in rock crystal (quartz), found at Minas Gerais, Brazil. The irregular cavities formed by growth contain remains of the aqueous solution in which the rock crystal grew about 500 million years ago. The bright interference colours of the otherwise colourless quartz are formed by the almost parallel viewing direction to the optical axis of the crystal in polarised light. Width of image: approx. 6 mm, transmitted light, crossed polarisers.

The Main Gemmological Examination Methods

As a general rule: Gemmological examination methods must not be destructive.

Standard equipment

  • 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 IR (exact measurement of absorptions in the UV to visible light range and in the infrared range)
  • X-ray fluorescence (XRF): semi-quantitative analysis of trace elements
  • Raman spectroscopy: analysis of molecular structures (e.g. determination of inclusions)
  • Laser-ablated – inductively coupled plasma – mass spectrometry (LA-ICP-MS): highly sensitive trace element analysis
  • Scanning electron microscopy (examination of submicroscopic surface structures)
  • Laser induced breakdown spectroscopy (LIBS): further trace element analysis
Fig. 6: Inclusion of a fly in Baltic amber. Amber is a fossil resin, and this sample originated about 35 – 40 million years ago in forests in what is now the Baltic region. Amber occasionally contains inclusions of insects, spiders, small vertibrates, etc., which stuck to the resin and were covered by subsequent resin layers. Thus they have been excellently preserved. After the tree died, the lumps of resin were washed into rivers and then into the sea, where they were embedded in sediment and fossilised. Although amber is an amorphous substance and in theory optically isotropic, the flow structures of the resin due to internal strain as well as the strain caused by the inclusions can be visualised in polarised light. The use of polarised light and the first-order red compensator lead to intensive colours in the otherwise golden amber. Combination of darkfield and incident light (glass-fibre), crossed polarisers, red-1 compensator, HDRI tone mapping.