Feature Gems & Gemology, Fall 1995, Vol. 31, No. 3

Gem-Quality Grossular-Andradite: A New Garnet from Mali

A find of new gem garnets has been made in contact metamorphic deposits in the Republic of Mali, western Africa. These garnets, primarily yellow-green to brown but also (rarely) intense green, have compositions between grossular and andradite. Although garnets of similar composition are relatively well known, this is the first documented occurrence of gem-quality material in commercial quantities. Because this gem variety is not represented by any of the gem garnet terms previously in use, it is described as grossular-andradite by the GIA Gem Trade Laboratory. It can be distinguished from grossular by its absorption spectrum and (usually) higher refractive index; it can be distinguished from andradite by its lower R. I. The stacked parallel planes of growth zoning, always visible between crossed polarizers, are diagnostic of grossular-andradite.