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Sierra de Cruces, Coahuila, Mexico

The state of Coahuila is situated in the north of the Mexican Republic, bordering on its north and northeast limits with the United States of America. It occupies a portion of the northeast Mexican high plateau and the northern parts of the Sierra Madre Oriental. The word ‘Coahuila’ means ‘the place where the GUACHICHILE trees and the TOBAGOS grow’. Even though Alvaro Nuñez Cabeza de Vaca was the first European to set foot in this desolate territory inhabited by unnamed nomadic tribes of Chichemeca Indians, it was not until 1568, that explorers began searching the Valle de Saltillo for mines. The principal source of income was from the capture of the Chichimeca Indians, which were sold for labouring in the mines.

Grossular from Mexico

The terrain of this region is very irregular on account of the Sierra Madre Oriental that runs across its entirety from south to north in its central-oriental region. As a result there is a complex of hills and mountains throughout the region with interesting names. One of these mountains called Sierra de Cruces (The Mountain of Crosses) which consists of an alkaline vault with rocks of limestone and dolomite dating back to the Cretaceous Era. These mountains are rich in calcium carbonate, which gave rise to the blossoming of grossularite, calcite, quartz, wollastonite and vesuvianite. In some specimens calcite can be found in the crystals of grossularite.
The larger these crystals are the more uniform their colour is, the smaller crystals exhibiting a variety of shades from ochre to scarlet. An analysis of the composition of the red garnets reveals that they owe their red colour to a high concentration of manganese, and their darker centres (hearts) to even higher concentrations of iron. Today there are doubts as to whether or not the mines will continue to provide specimens of grossular, as their extraction is complicated and their cleaning difficult on account of the hardness of the matrix. Nevertheless, the miners carry on extracting them. A local rancher, in the mid 1970’s came across an interesting carmine red stone. He showed it to a friend who had some knowledge of minerals and he immediately identified it as grossularite. With this discovery the rancher applied for a permit to explore the area that turned out to be the bearer of a wide variety of multicoloured specimens. They ranged from the deep blood red of grossularite to the fiery yellow of the vesuvianite. The specimens from this area are of particular interest on account of the way the crystals overlap each other creating amazing geometric figures.
The grossularite mine is a small cavity that opens up at the side of a stream less than a kilometre from the Sierra de Cruces. The entrance is no more than four metres wide and opens up to the outcrop of grossularite; this combined with vesuvianite cover half of the exposed rock surface. Calcite spreads throughout the skarn in the form of white grains that are associated with quartz and wollastonite.