Rocks are naturally occurring solids which are composed of minerals & have been used by humans since ages. From Stone Age, rocks are used for various purposes. Also, the metals and minerals found in rock play an important role in our life. Get to know all the Hornfels Uses. We have provided you with all information about Hornfels rock here. Hornfels is a metamorphic rock formed by the contact between mudstone or other clay rich rock, and a hot igneous body, and represents a heat altered equivalent of the original rock. Hornfels is available in brown, dark greenish - grey, green, reddish brown colors. The streak of a rock is the color of powder produced when it is dragged across an unweathered surface. The streak of Hornfels is unknown. Get to know more about Hornfels rock and characteristics of Hornfels rock in the next sections.