Inclusions in precious and semi-precious gemstones

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).

Fig. 1. Some spectacular examples of mineral inclusions: upper left – liquid and gaseous CO2 in a sapphire; upper right – quartz crystal in a diamond; lower left – rutile, calcite and apatite in a ruby; and lower right – rutile needle and quartz in a garnet.

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 100 µm 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.

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