corundum, synthetic corundum.7, flux, Knischka, gems

Gunawardene examines the gemmological and crystallographic characteristics of the newly developed Knischka synthetic ruby, produced by Paul Otto Knischka of Austria using a modified gradient-transport technique involving supercooling and supersaturation. The paper describes the unusual crystal morphology of the material, including the development of numerous additional crystal faces, and compares these forms with those of natural corundum. Particular attention is given to microscopic identification. Characteristic internal features include turbid clouds, swirls of colour, liquid or feather-like patterns, negative crystals associated with elongated crystalline tubes, metallic platelets, melt residues and two-phase inclusions. Although some inclusions can resemble features in natural ruby or other synthetic rubies, their morphology and associations provide useful criteria for recognizing Knischka material. The study emphasizes microscopy as the principal means of identification and illustrates the diagnostic inclusions and crystal-growth features in detail.