Hornfels forms when sedimentary or volcanic layers are baked from 300-800º C by an intruding mass of igneous magma, but at a relatively low pressure.  This contact metamorphism fuses the layers together and makes the stone suitable for flaking into tools.  Hornfels is usually dark brown or black.  Variants of the stone can resemble chert, although cherty hornfels is usually more coarse in texture.