gems, garnet, pyrope-almandine, rhodolite, heat treatment

Although garnets are generally believed to be unaffected by treatments like heating or irradiation and thus are typically considered untreated, Gerhard F. A. Becker of Idar-Oberstein challenged this assumption with heat-treatment experiments on uncut Brazilian rhodolite garnets. Using a standard muffle furnace in air, he incrementally heated the purple garnets in 50°C steps, observing the first visible color change at 700°C, with continued alteration through 900°C. At higher temperatures, a metallic, hematite-like coating formed on the stones, which could be polished off. Becker’s findings confirm earlier preliminary work by Dr. Kurt Nassau, who, following an observation by G.V. Rogers, altered purplish rhodolite to a hessonite-like brown at around 600°C. The metallic coating was likely due to iron oxidation from heating in air and may resemble a similar phenomenon reported in Gems & Gemology (Fall 1975), where an almandine garnet was mistaken for cuprite—though at that time, the effect was believed to result from sputter coating rather than heat treatment.