Slovenia boasts the world’s second-largest mercury deposit, located under the historical town of Idria. The most important ore mineral was cinnabar and only to a minor degree native mercury. The ore field extends along the Idria Fault zone over a relatively small area of 0.6 km2. In a 500-years long mining history, more than 700 km of tunnels and seven shafts have been dug to access the ore bodies and transport the ore to the surface. Deep drilling revealed that the ore extends to 450 m below the surface. With 15 levels, a depth of 382 m was reached with the deepest tunnel plunging 36 m below sea level. The Idria ore deposit has a very complex tectonic structure. Ore mineralization occurred in two consecutive phases in the Middle Triassic. In this process, all the rocks from Carboniferous to Triassic were mineralized. Due to intense tectonic activity in the Tertiary, the mineralized rock sequence was brought in an inverse position with Carboniferous rocks sitting on top of the younger beds. By far the richest are the interstratified ore bodies, which are mainly of synsedimentary origin, while some were formed by metasomatic replacement of the soluble layers in Triassic limestones. Mineralogically the most interesting were poorly mineralized discordant ore bodies, occurring in tectonically shattered zones along the steep faults that served as conduits for ore solutions. In these zones, fissures with up to 50-cm large cavities covered with crystals of dolomite, quartz, calcite, baryte, pyrite, cinnabar and other accessory minerals were encountered during mining. For mineralization of the ore deposit was the presence of hydrocarbons in the bedrock was crucial. In the process of pyrolisis, polycyclic hydrocarbons formed and the sulfur needed for cinnabar formation was released. Hydrocarbons influenced the crystallization and they occur in form of bitumen alongside the ore minerals throughout the deposit. Until today, 25 minerals were identified in the Idria deposit. For two of them, siderotil and idrialine, Idria is their type locality. Ruby red crystals of cinnabar from Idria are found in all major mineralogical collections and museums around the world.