Grossular from the Jeffrey Quarry, Quebec is one of our Top 100 Minerals.
Grossular is a calcium aluminum silicate. It is a member of the garnet group which is dominated by six species: almandine, pyrope, spessartine, grossular, andradite and uvarovite. Hessonite is a cinnamon-colored gem variety of grossular.
While the main ore at the Jeffrey mine is asbestos---fibrous serpentine group minerals, such as the chrysotiles, antigorite and lizardite---the locality is primarily known to collectors as the producer of some of the world's finest and gemmiest grossular and vesuvianite. These minerals are found in a rock known as rodingite, a grossular and diopside dominant rock produced when granitic dikes hosted by iron- and magnesium-rich rocks are altered by calcium-bearing hydrothermal solutions. At the Jeffrey mine, the main orebody is a package of ultramafic rocks and is cut by granitic dikes. It is at the contact of these two rock types that the rodingites formed during the serpentinization of the former.
Specimens of grossular from this locality are commonly associated with diopside, unusually distinct prehnite crystals and acicular pectolite.
