Hornfels is a contact metamorphic rock.
It is known for being hard to
identify, mainly because it can form from any rock and consequently
have many different appearances.
It most often forms from fine grained
rocks such as andesite,
basalt, dolerite, limestones, shales
How it looks like depends on the original rock and the
temperature at whch it was formed. It forms at very high temperatures
and low pressures.
By .RGB. via Flickr.com
It is usually hard, dense, fine-grained,
homogenous, not foliated, but may be banded.
It has a concoidal,
flintlike fracture, which gives it the broken, horn-like endges.
By Ian Geoffrey Stimpson via Flickr.com
very fine-grained matrix can contain porphyroblasts or poikiloblasts.
It is usually black, grey or bluish in colour.
It often contains
minerals such as micas, garnets,
sillimanite, pyroxene, cordierite,
andalusite and others.