OK, thanks all. I didn't message anyone at CMON yet, since I assumed it was an obvious technique with an obvious answer (my basing knowledge is really bad). But it looks like there are a number of possibilities. I recall reading something like what Omenbringer described (breaking a pancake of some kind of clay) but thought that was for more general-purpose rocks than flat jaggies. But I suppose if you have a zillion pieces, then you'll find at least one that works.