corundum, color.4, gems

Spectrochemical and spectrophotometric investigations of natural and synthetic rubies and sapphires were undertaken to compare their compositions and optical properties and explore possible methods of distinguishing them. For spectrochemical analysis, ruby samples were ground in a boron carbide mortar under acetone to minimise contamination and examined using the Harvey method of semiquantitative analysis. Results for two natural and two synthetic rubies, comprising a light and a dark specimen of each, were expressed as oxide percentages. Alumina constituted 96.5–98.8% of the samples, while chromium oxide ranged from 0.945 to 2.58%, with higher concentrations in the darker specimen of each pair. Silica and minor quantities of copper, cadmium, magnesium, molybdenum and iron were reported in all four specimens. Vanadium was detected in both natural rubies but in neither synthetic ruby. Numerous additional elements, including titanium, were sought but not detected in these four samples. Separate analyses examined the clear core, combined core and outer layers, and outer layers alone of a Linde synthetic star ruby to investigate the material producing asterism. Titanium increased from 0.064% in the core to 0.162% in the outer layers containing dense interlocking crystals. This enrichment supported the suggested identification of the star-producing inclusions as titania, consistent with their microscopic resemblance to rutile needles in natural rubies. Magnesium was also more abundant in the outer layers, whereas chromium concentrations were relatively similar throughout the analysed sections. Spectrophotometric measurements were made with a Hardy recording spectrophotometer on highly polished plates of natural and synthetic ruby and sapphire measuring approximately 2 mm square and 1 mm thick. Visible spectral reflection curves showed broadly similar patterns for natural and synthetic examples of each gem variety, in keeping with their closely comparable chemical and physical properties. Ruby curves exhibited a broad blue-region maximum, a yellow-region minimum and a pronounced rise towards red wavelengths; sapphire curves showed a broad blue-region maximum followed by lower reflection at longer wavelengths. The curves were submitted to Eastman Kodak for investigation of a special filter that might permit rapid differentiation of natural and synthetic corundum. Research remained incomplete, and no wholly satisfactory filter had been obtained.