synthetic spinel, Verneuil, gems

E.J. Gübelin investigates whether synthetic red spinel was commercially available, correcting a statement in A.K. Seemann’s 1949 account of American synthetic crystals that listed ruby-red and garnet-red spinel among available colors. Literature review and correspondence with W.F. Eppler establish a distinction between the experimental possibility of producing red spinel and the practical manufacture of transparent, durable material suitable for cutting and sale. Gübelin explains that conventional Verneuil production of single-crystal spinel requires a substantial excess of alumina over the equimolar proportions of magnesium oxide and aluminum oxide represented by natural spinel’s formula, MgAl₂O₄. Attempts using the natural proportions instead produce aggregates of differently oriented crystals that crack readily on cooling. Commercial synthetic spinel may contain approximately 3.5 moles of alumina per mole of magnesia. This compositional difference affects the colors produced by added metal oxides: a coloring agent need not impart the same hue to spinel and corundum because their crystal structures differ. Chromium, responsible for red ruby and natural pink spinel, produces dark green in the alumina-rich synthetic spinel under discussion. Gübelin reviews the characteristic absorption and fluorescence associated with chromium and describes contrasting thermal behavior in ruby and green synthetic spinel. Ruby becomes green at approximately 500–600 °C but recovers its original red color on cooling. Green synthetic spinel undergoes precipitation of excess alumina at approximately 950–1050 °C, becoming cloudy; above about 1075 °C, structural changes can produce red coloration accompanied by deterioration of transparency and gem quality. The article interprets these effects through the behavior of chromium-bearing alumina within different crystal structures. M.T. Mackowsky’s published observations and Eppler’s experimental experience agree that red synthetic spinel can be obtained when magnesia and alumina are maintained near a 1:1 molar ratio, but the resulting specimens are generally tiny, cracked, or shattered. Eppler reports that increasing the alumina content toward the proportions needed for satisfactory commercial boules immediately changes the color from red to green. Gübelin therefore concludes that red synthetic spinel is producible in principle, while satisfactory cutting material had not been established as a commercial product at the time of writing. He regards Seemann’s listing as an overlooked error, while allowing that future technical advances might overcome the manufacturing difficulties.