Photomicrography > Corundum
First draft: May 15th, 2014 | Last update: July 27th, 2014
Inclusions in Corundum
Inclusions in rubies and sapphires have provided conclusive information about their genesis since the 1950's. In those days, determining the origin of a ruby or a sapphire, which from then on has had a repercussion on its monetary value, was almost purely a matter of appearance and inclusions. But since then, the number of deposits has almost tripled and things changed for good.
The often similar yet sometimes particular geological setting and consequent paragenesis of these new deposits called for additional methods in order to discriminate between the traditional geographical sources and the newer ones. Nevertheless, some specific inclusion scenes may still prove quite adequate for positive determination.
Besides crystal inclusions, internal growth features (in the form of colour zoning) are reminiscent of the original crystal habit and may thus also be an additional aid contributing to resolving a gem's origin. This is particularly true for combinations of faces of the hexagonal dipyramidal crystal shapes - especially those found in gems originating from Sri Lanka and Madagascar.
While microscopic examination of rubies and sapphires still provides valuable information useful for origin determination, optical identification of inclusions is sometimes not possible. Therefore, highly specialised analytical, preferrably non-destructive techniques have become indispensible in today's gemmological laboratory where they fulfill an increasingly important role.
An extremely comprehensive overview of corundum geology widely covering classification, genesis of deposits, geochemistry, radiometric dating, and oxygen isotopic composition is given by Guiliani et al. (2014). It sheds a highly informative light on the vast complexity of corundum origin by reviewing all major studies carried out on the subject and adding extra information.
Besides the issues encountered with origin, treatments have also altered dramatically - and not only within the corundum family. The traditional heat treatment or bâta kumbala, a method known for centuries to alter the appearance of rubies and sapphires, is now complemented with diffusion techniques, fracture filling by means of flux or glass, and completely reconstructed stones.