In first semester I took an introduction to lithography course. The first thing we had to do was grain our stone. One way of graining bavarian limestone is to use two stones, one large (about 3 feet by 4 feet) and one smaller (10 x 12 inches). Water and grit is placed on the large stone and the smaller stone is place on top of the large one (drawing sides together). By sliding the smaller stone over the larger one repeatedly, using progressively finer grit and all the while alternating the pattern of your motions frequently the two stones wear away at each other till both are even, smooth and ready to be drawn on.
This process is not the most efficient way of graining a limestone, but since the graining is so gradual the chances of damaging your stone or creating an uneven surface diminishes.
Some students worked for hours graining their stones only to discover that the 320 grit (the finest and last grit) was not the silky carborundum it appeared to be. Within the grit were large sharp somethings (we still don't know what they were) and when these sharp somethings got wedged between the two stones they were driven into the surface of the stone causing scratching... After something like this happens you need to start over. 4+ hours of work gone.
So you start again and remember to avoid the 320 carborundum grit!