You'd probably want fine sand - not coarse. But I think even the finest sand you could buy at a big-box home improvement store might be too coarse, and/or have

You'd probably want fine sand - not coarse. But I think even the finest sand you could buy at a big-box home improvement store might be too coarse, and/or have impurities in it. So you'd be left with getting certain purified "sands", that would be closer to being called "cutting agents for water jet cutters" than anything else. With the associated ($$$) pricing. But maybe there's something cheaper out there? Maybe there's some kind of finer mortar grit additive that isn't too expensive that would work?