corundum, ruby, sapphire, inclusions.5, Kashmir.12, India.12, gems

Schubnel examines the identification of solid inclusions in gemstones through laboratory techniques that supplement microscopic observation, emphasising their value in establishing natural origin and, in some cases, suggesting a probable geographical source. Reporting results obtained during the preceding two years, he explains that inclusions around one tenth of a millimetre or smaller need no longer present insurmountable identification problems when suitable mineralogical equipment is available. The choice of method must nevertheless be adapted to the individual stone, and the general inability to subject cut gemstones to destructive testing imposes practical restrictions. Direct determination is confined to inclusions that reach the surface; in rough crystals, polishing a face may be necessary to expose the inclusion. Three complementary approaches receive particular attention. Chemical analysis with the Castaing electron microprobe uses characteristic X-ray emission to establish the composition of very small inclusions. Microscopy in polarised reflected light provides optical observations and measurements, particularly for opaque minerals. X-ray powder diffraction can identify crystalline material removed from inclusions in rough stones, while measurements with a Leitz microhardness instrument leave square impressions only a few micrometres across. Illustrated identifications include pyrrhotite and chlorapatite in a mauve sapphire, a spinel containing 3% zinc in a cut ruby, and calcite in ruby. A more detailed example, presented in French, English and German as a suggested format for communication among inclusion specialists, describes calcite inclusions in a faceted ruby of presumed Burmese origin. Examination in transmitted light with the ruby immersed in methylene iodide reveals a large inclusion whose faces resemble a nail head and whose cleavage planes are clearly visible. Because this inclusion reaches the surface, its identification could be investigated using the Castaing microprobe, reflected-light microscopy and microhardness testing. The article anticipates increasing difficulty in distinguishing natural from synthetic gemstones and proposes systematic investigation of inclusions, particularly through microprobe analysis, as a means of improving the confidence with which gemmologists and mineralogists recognise and distinguish inclusion types.