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.

Nephrite and jadeite are green metamorphic rocks often referred to colloquially as ‘jade’.  Nephrite differs in crystal structure to jadeite.  Nephrite is known as a ‘amphibole’ mineral with oblique cleavage planes of about 120 degrees.  Jadeite is a ‘pyroxene’ mineral with cleavage planes of about 90 degrees.  Nephrite is less dense than jadeite and has a less vitreous lustre.  Both minerals are exceptionally tough because they are made up of interwoven fibre-like crystals.  Because of their crystal structures, nephrite and jadeite are unsuitable for flaking.  However, they can be shaped by sawing and grinding, followed by polishing.  Nephrite and jadeite were highly desired for their green colour and these stones had sacred connotations in many parts of the world.  The materials are perhaps most familiar as sculptures and jewellery, but stone axes were often made from them.  Their exceptional hardness allowed the materials to be grounded and polished to an exceptionally sharp cutting edge.