The Hunsrück area is a low hill country with a very long geologic history. The oldest rocks are 1Ga old, from the Precambrian. Ordovician and Silurian sediments were metamorphically changed into paragneiss.
The most important era for the Hunsrück was the Devonian. At the Lower Devonian the Kaledonian Orogeny to the northwest was eroded. The Rhinean syncline, a huge bowl with a downward movement, was the place where the sand and clay was sedimented again. The result are iron rich red conglomerates, sandstones, quartzites, and slates. Most important is today the typical Hunsrück slate. During the Middle Devonian volcanic activities started, formed a submarine ridge which was soon used by a reef. The reef limestones are called Stromberger Kalke, and are up to 400m thick.
During the Carboniferous, while nearby huge layers of coal were deposited, the Hunsrück was an area of hydrothermal activities underground. Hot water circulating through tectonic clefts was soluting minerals depositing them again in any cavity. The clefts were filled with copper rich ores, which were mined during the Middle Ages.
During the Permian, red and green sandstones were deposited, but most important is the heavy volcanism around Idar Oberstein. Huge layers of basalt were formed, with huge gas bubbles inside. The caverns were again filled by hydrothermal activity, caused by the heat of the volcanism. The bubbles were filled by semiprecious gemstones, varieties of quartz and other minerals.
The three economically most important deposits of the Hunsrück are the slates of the Devonian, the copper ores of the Carbon, and the Permian gemstones. All were mined, but are of now economic value any more. There are show mines for all three. Only the gemstones are important until today, as Idar Oberstein has the most imoprtant production of high quality jewelry in Germany. Obviously the gemstones which are used originate from all over the world, but not from local mines.