synthetic spinel, gems

E.J. Gübelin reports successful production of ruby-red synthetic spinel by an adapted Verneuil process, revising his earlier conclusion that satisfactory cutting material was practically unobtainable. The investigation examines six faceted specimens ranging from 0.37 to 2.00 carats, with colors varying from metallic purple-red to an attractive ruby red. Gübelin attributes the advance to precise control of highly purified starting materials, an equimolar mixture of magnesia and alumina, chromium oxide as the coloring agent, and closely regulated temperature and material supply. Although the boules remained partly cracked and yielded relatively few flawless portions, they represented single crystals rather than aggregates. X-ray powder diffraction patterns of natural and synthetic red spinel corresponded closely in line positions and intensities. Measured lattice constants of approximately 8.084 Å for the synthetic material and 8.086 Å for the natural comparison confirmed their structural similarity but offered no practical means of distinguishing origin. Specific gravities ranged from 3.579 to 3.598, and refractive indices from 1.7191 to 1.7198, closely matching natural red spinel. The absorption spectrum likewise resembled that of natural material, with broad absorption extending approximately from 490 to 595 nanometers. Fluorescence provided a more useful distinction: the synthetic specimens showed dull red fluorescence under short-wave ultraviolet radiation and a brilliant red response under long-wave radiation. Their fluorescence spectrum contained a strong broad red band at approximately 685–690 nanometers and a faint satellite line near 678 nanometers, contrasting with the multiple lines described for natural red spinel. Microscopic examination supplied the clearest evidence of synthesis. Gas inclusions included spherical, distorted, elongated, and tadpole-shaped cavities, with minute bubbles concentrated into dense clouds or broad parallel bands. Distinct curved striae and growth bands occurred throughout the series, including in specimens lacking conspicuous bubbles. These features resembled those of synthetic corundum more closely than those commonly observed in conventional alumina-rich synthetic spinel. Gübelin relates their conspicuous development to difficulties in maintaining steady growth. Extinction between crossed polarizers varied with internal perfection: heavily included specimens displayed strong anomalous birefringence with feathery or banded textures, whereas cleaner stones showed complete or nearly complete extinction. He cautions that the combination of bubbles and curved bands could initially suggest synthetic ruby, making the absence of dichroism and examination of optical behavior important. The article establishes synthetic red spinel as a new identification problem while emphasizing that routine physical properties alone may closely reproduce those of natural material. Gübelin concludes by urging manufacturers to develop distinctive decorative materials whose appeal does not depend on resemblance to natural gems.