Any gem and mineral book should discuss cleavage planes. the Audubon
field guides for instance have a brief discussion of the various
types of cleavage relative to the gem and mineral kingdom replete
with diagrammatic arrows. So get out a cold chisel with a 1 inch face
and a good dead blow hammer and put on your eye protection, take it
outside and place the stones on a stump, or other absorbent
"pedestal" for lack of a better word and hit the material with enough
force just to break it on the plane indicated, but nowhere near
enough force to get your aggressions out ( if there are any!). You
may wrap the stone in a towel (once aligned within it -then strike
with the hammer) first to catch minute pieces. good luck…another
stone that may interest you is limonite after pyrite.
I would think the contrast has some design potential! RER.