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A massive, fine-grained, granoblastic contact metamorphic rock, commonly displaying conchoidal fracture and splintery debris. Heat from an igneous intrusion initiates recrystallization of surrounding rocks, thereby producing a contact metamorphic hornfels which is usually finer-grained than the country rocks. Hornfelses formed by recrystallization of shales, and occurring away from the contact with the igneous body, quite often develop prominent graphite-rich spots formed by the aggregation of organic material in the original shales. — A Dictionary of Geology and Earth Sciences (4 ed.), Oxford University Press