At present when you knap stone you can just cut out around immediately close to the final shape and it looks like you should be able to pick it up and leave the remaining stone on the floor. I think it would be better if to begin with you only had the edge of the square showing yellow voxels and as you chip away they expose more edges and then more yellow as appropriate.
Just quickly thinking it through I would have thought that making it have rules along the lines of you can knap if the outer voxel is exposed or the two on either side are either exposed or could be exposed. I have attached a quick diagram of how I see it working, but haven't checked that it would definitely would work without errors.
May need a bit more refinement but hopefully would be a bit more like how one would knap stone in real life.