Dr Robert Sturm (Austria)
Gemstones are commonly regarded as natural wonders, and their infatuating beauty and rareness has fascinated us from time immemorial. Besides the obvious macroscopic appearance, many a gemstone is characterised by a ‘hidden’ microscopic inner life of breathtaking aesthetics. Among non-experts, such inclusions in precious and semi-precious gemstones are often interpreted as ‘pollution’ or as ‘blots’. On the other hand, among gemologists, inclusions bear valuable information about the genesis of their hosts and may also increase the value of a stone.
The main characteristics of inclusions in precious and semi-precious gemstones
Basically, inclusions in gemstones occur in three aggregate states: solid, liquid and gaseous. Solid inclusions are generally represented by those minerals found in close vicinity to the host stone or correspond with the chemistry of the host stone. These mineral inclusions either crystallise before their host (protogenetic), at the same time (syngenetic) or after its formation (epigenetic). Epigenetic crystallisation of inclusions takes place in most cases by so-called dismixture processes during the cooling of the host stone. Inclusions being generated in such a way are commonly characterised by the same orientation as the host crystal (for example, needles of rutile in corundum – rutile is a mineral composed primarily of titanium dioxide).
Liquid and gaseous inclusions are often marked by some kind of coexistence, so that they are summarised by the term “fluid inclusions”. They have to be regarded as a consequence of the fact that many gemstones form from a liquid or aqueous medium, and tiny blebs of that liquid can become trapped within the crystal structure. Fluid inclusions range in size from about 100µm to 1mm and are therefore easily identifiable under a light-microscope. Besides their aesthetics, fluid inclusions may also contribute to the solution of various scientific problems: including (among other things) the understanding of the role of fluids in the deep crust and crust-mantle interface, and the reconstruction of climate conditions during the formation of the host mineral.
Some examples of inclusions in various gemstones
Some spectacular inclusions worthy of being considered in detail under the microscope are arranged in Fig. 1. The first example shows a blue sapphire from Mogok/Myanmar, bearing an embryo-like inclusion filled with liquid and gaseous carbon dioxide (CO2). The upper right margin of the inclusion is further characterised by a small platelet of graphite with a clearly recognisable hexagonal habit. In the next image, we can see the interior of a diamond, including a wonderful quartz crystal with near-perfect symmetry. While the host stone has only a diameter of 1.2mm, the quartz inclusion itself measures 0.2 x 0.07mm.
The image on the lower left exhibits a thermally untreated ruby from Mogok/Myanmar, with an inclusion characterised by several mineral species. In the upper part, we can observe extremely fine needles of rutile, whereas the right part is marked by very regularly formed rhombohedra of calcite. However, the most spectacular mineral phase is an extended apatite crystal, reaching from the lower left to the upper right corner of the image (length: about 2.5mm). For a gemologist, such an inclusion is sure evidence that the host stone has not been heated to artificially improve its colour.
The last image on the lower right shows an alien-like inclusion in a garnet from Madagascar, which measures about 1mm in length. As can clearly be seen, the inclusion consists of two distinct mineral phases: a long and extremely thin needle of rutile and a mostly irregularly crystallised quartz crystal.
The second collection of inclusions arranged in Fig. 2 is surely no less spectacular than the first. In the upper left image, we can see a quartz inclusion in a garnet resembling the “S” of the superman emblem. The twinned quartz measures about 0.50 x 0.13m and is surrounded by extremely fine needles of rutile. The image on the upper right shows an impressive example of an epigenetic inclusion (see above).
The inclusion is rather similar to a millipede and consists of tiny fissures formed during the cooling of the host mineral – it is a moonstone. After their formation, the fissures were filled with albite and orthoclase. Another eye-catcher is the image on the lower left, showing very fine tentacles of pyroxene crystals in an obsidian. The formation of such bizarre inclusions in a volcanic glass is a very rare phenomenon. The length of the tentacles is in the order of 1mm.
The last example in this figure exhibits a spider belonging to the genus Salticus (2 x 2mm) that was captured in resin 35Ma. Over millions of years, the resin hardened to amber. The most impressive of such ambers occur in the Dominican Republic and along the coast of the Baltic countries, as well as in Madagascar and Columbia.
A mineral commonly bearing a high number of all kinds of inclusions is zircon, which can be separated from igneous and metamorphic rocks. In Fig. 3, four zircon crystals (the small images) and their main inclusions are exhibited. These (among others) include long crystals of apatite (upper left image), rutile (lower left and lower right) and quartz crystals (lower right), as well as smaller crystals of zircon that were formed during earlier processes of magmatic crystallisation (upper right).
Since, in most cases, zircon crystallises from a magmatic source, inclusion phases may help us to understand those physical and chemical processes taking place during crystal formation and, more generally, during granitic rock formation. For instance, inclusions are important for decoding the sequence of magmatic crystallisation, that is, which mineral crystallises first and which last. Furthermore, some inclusions may be used as geothermometers for an evaluation of the temperature of the magmatic melt.
The main objective of this small contribution is to demonstrate the aesthetics of inclusions occurring in diverse gemstones. Such inclusions are often very typical for specific host minerals and can effectively contribute to the questions of whether or not their hosts have been treated thermally to artificially improve how they look. Furthermore, inclusion phases serve for a reliable distinction between natural and artificial gemstones, the latter of which do not include any crystals.
Hollister, L.S.: Fluid inclusions: applications to petrology. Mineralogical Association of Canada, Calgary, Alberta 1983.
Roedder, E.: Fluid inclusions: an introduction to studies of all types of fluid inclusions, gas, liquid, or melt, trapped in material from earth and space, and their application to the understanding of geologic processes. Mineralogical Society of America (MSA), Washington 1984.
Weibel, M.: Edelsteine und ihre Mineraleinschlüsse. ABC-Verlag, Zürich 1985.