Precambrian metamorphic and igneous rocks are the hardest rocks in this region. Because they don’t erode quickly, they make up the high peaks of the Front Range. Beautiful pink Pikes Peak granite is a common rock type in this group. It formed as the magma chambers beneath volcanoes cooled and crystallized over 1.2 billion years ago. The magma intruded into a terrain of the metamorphosed sedimentary rock that you see along I-70 at Idaho Springs—thus this formation is called the Idaho Springs Formation. The Precambrian sediments that made these rocks have been transformed into mica-rich schist and gneiss, hosts for a variety of ore deposits that are mined nearby. The Pikes Peak granite also contains veins of crystals, including the lovely sky blue feldspar called amazonite.