This is a prismatic core. It’s the centerpiece of a fairly advanced stone tool making technique. Long, narrow, extremely sharp blades can be flaked off of this core in a clockwise fashion, slowly narrowing down the core until it gets about this small, where it would be discarded, and a new one prepared. From one single core, dozens of individual blades can be removed.
via Flickr http://flic.kr/p/eefpoL