The cabochon in facts as a biconvex lens, which focuses the star image above the gem. The star image results from the incoherent superposition of reflected Frauhofer diffraction patterns, which arise when light is scattered around the network of inclusions. In corundum, due to the three-fold rotational symmetry of the basal pinacoid, networks of exsoluted syngenetic inclusions are oriented in three different crystallographic directions that intersect at 120°. What is asterism?Īsterism is the star effect seen by reflection and scattering of light in cabochon-cut stones with suitably oriented rod-like inclusions or platelets. Other frequently encountered oriented oxide inclusions are hematite (Fe 2O 3) and ilmenite (FeTiO 3). Read more: Sri Lanka - A Gem of an Island How does the exsolution process of rutile occur in star corundum?Īt temperatures of 1200-1400☌, titanium moves through the resulting expanded crystal lattice and between the oxygen atoms, exsolving into rutile needles. In most cases, it occurs upon cooling below the temperature of mutual solubility. What is exsolution?Įxsolution in mineralogy is a process where an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any material. Solid inclusions are divided into three categories, by time of entrapment: those formed before the host crystal, called protogenetic solids which arise from the solution from which both they and the host originated, called syngenetic and those formed after the host crystal has finished its growth, epigenetic. This article by Guy Lalous ACAM EG summarises a technical article from The Journal of Gemmology discussing a giant 12-rayed black star sapphire from Sri Lanka with asterism caused by ilmenite inclusions.
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