Hornfels are a group of hard, fine-grained common, metamorphic rocks formed originally in a high-temperature area in near contact with an igneous intrusion. They all contain quartz and mica, with a third component that gives then their individual names: chiastolite hornfels, cordierite hornfels, garnet hornfels, and pyroxene hornfels. All are dark gray or black, although garnet hornfels has reddish patches.