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.

Jadeite is the mineral that composes jade.  It is extremely tough and coarse because of the way the  crystals interlock, and jade is unsuitable for flaking or pecking.  However, it can be shaped by grinding and, because of its hard texture, it can be smoothed and polished.  Jade was highly desired for its green colour and had sacred connotations in many parts of the world.  It is perhaps most familiar as sculptures and jewellery, but stone axes were often made of jade.  Jade’s hardness allows it to be grounded and polished to an exceptionally sharp cutting edge.  Nephrite—a different form of jade composed of nephrite crystals—has a less vitreous lustre than jadeite.