I did a bit of noodling around with various different pieces of stone and a striker last winter. (I wouldn't go so far as to call it an experiment.) My conclusions were that although almost anything with an edge will work to some degree, you don't want too sharp of an edge, as it'll chip too easily, and you don't want too small a blade as it's not easy to hold onto and it allows you more edge to use before you must retouch or get another piece. I found a steep-retouched edge best, striking against the flat rather than the bevel. (I was using steel rather than pyrites.)
