Hornfels is a metamorphic rock formed when any clay-rich rock such as mudstone or shale comes into contact with a hot igneous body. Hornfels is the name given to the heat-altered version of the original rock and may originate from multiple varieties of rocks. The process by which hornfels is formed is called contact metamorphism.
While many metamorphic rocks contain foliation due to the high pressure and temperatures present during their formation, neither are present during the formation of hornfels, so foliation is absent. As hornfels is formed by heat, most pre-existing bedding and structure of the parent rock is destroyed during creation. Hornfels typically has a granular texture and is very fine-grained.
According to the Geology department at The University of Auckland, hornfels may be difficult to identify without microscopic assistance. However, when observed under a microscope, hornfels has a very distinctive shape as its grains are equigranular and thus all roughly the same size. It is a hard rock, and its color and mineralogy are often variable, as they depend on the characteristics of the parent rock. The most common type of hornfels is the biotite hornfels, and it is normally a dark brown or black color. Hornfels is mainly used in construction or road-building industries as aggregate.