The valley of Sainte-Marie-aux-Mines is situated on a major geological fault network. Over the course of time, the majority of the rock, gneiss, has been subjected to the effects of runoff. The metalliferous materials (silver, lead, copper, etc.) it contained dissolved and accumulated on the walls of the faults. This gave rise to the silver-bearing veins, which were exploited from the Middle Ages until the middle of the 20th century. More than 1,100 mines were dug in the region during this period. The silver ore in the Saint-Jean Engelsbourg (St. John Engelsbourg) mine (photo) is in the form of tetrahedrite (a mixture of copper and silver) or native silver.