Buy a few small samples. It should be cheap. Even if you had to buy a few square feet, it would still be cheaper per point than most stone. If you have not knapped any slab material, it would be good to study a tutorial or watch a few videos on how to tackle it. Experiment. I have knapped a few pieces of floor tile, and it was fun to work and made nice points. I never knew what specific type it was, because I dug it up in the yard. The finish looked a lot like Patrick's, no gloss, and the flakes did not travel as well as good stone or glass, but it was still good stuff to work.
WA