A metamorphic rock that has been subjected to heating during contact metamorphism around intrusive igneous rocks. Hornfels is typically fine-grained, although where it is subjected to high temperatures, large crystals called porphyroblasts can form. In outcrop, hornfels is notoriously tough and can be difficult to sample. Mineral grains in hornfels are randomly oriented, with no preferred alignment of crystals to form foliation or cleavage planes. This texture indicates that the hornfels was not subjected to significant stresses during contact metamorphism.