Demantoid, Andradite, and Grossular from Mexico
Iridescent andradite garnet is an interesting phenomenal stone that was initially produced in Nevada but has also been found in Mexico, Japan, and New Mexico. A small parcel consisting of three stones (figure 1) was recently submitted to GIA’s Carlsbad laboratory and contained some new gem material from the iridescent andradite deposit in Mexico. The deposit is located high in the Sonoran Sierra in the Mexican state of Sonora about 145 km northeast of the city of Hermosillo. The stones were provided by Marion Alberto Márquez Suárez, who works with the mine owner, Salvador Barba.
Standard gemological testing gave a refractive index of 1.740 for the grossular garnet and over-the-limit readings for the demantoid garnets. All three samples were inert to both long-wave and short-wave ultraviolet light. Fingerprints and fields of fluid inclusions as well as colorless and dark crystal inclusions were observed (figure 2). The inclusion scenes were reminiscent of skarn-related demantoid garnet from Namibia and Madagascar rather than the horsetail inclusion scenes noted in serpentinite-related demantoid from Russia. Trace element chemistry was collected as part of an ongoing project for demantoid origin determination. Curiously, the demantoid garnets had low gallium levels ranging from 0.45 to 1.96 ppm. This is more in line with serpentine-related demantoid, with gallium less than 1 ppm, than with skarn-related demantoid, which always has gallium levels above 2 ppm. Chromium was also below detection limits, which is consistent with iron-colored demantoid from skarn-related deposits. While these Sonoran demantoid and other garnets are not currently on the market, the miners have produced up to 7 kg of the material over the last 10 years.
