David Weaver
you can calculate the quadrant of a square and the loss of volume in that square from the radius and then measure the length of the radius around the edges of the stone if it's consistent.

Of course, it's unlikely that it will be that regularly shaped, but you can estimate the error due to the radius by assuming it's 50% bigger than your measure and see the maximum volume reduction - it probably won't be that small.

I can't judge density of porous stones very well, but for black and trans stones, I can look at the surface and guess pretty well.