Here is a really intriguing, well-photographed post by Patrick Blank at Primitivearcher.com about shaping an arrowhead using pieces of sandstone. This inspired me to go out to my local landscape supplier and get a few hunks of sandstone to work with. It is essentially like having 80-100 grit sandpaper in a block. The natural irregularities of the stone made for some interesting surfaces to work the bone against. It would not necessarily be the first thing I would go to as he does, but would certainly be useful after general shaping with a rasp. You can apparently give sandstone blocks flat side if you grind two relatively flat pieces together (I haven’t tried) if you rub two blocks of it together. They are used in landscaping for building walls, so they come generally flat to begin with. You can also grind your piece against a flat concrete slab or piece of concrete sidewalk — which Jim Croft, tool-maker extraordinaire, told me he uses for making flat edges on his bone folders. The piece of bone I used on my sandstone has already entered the finishing stages (alas, undocumented), but next time I will definitely take some photos to share.