The stonemasonry skill is used in techniques called "Knapping" or "Pressure Flaking" which simply is using a stone on another stone to shape the other stone the way you want it.
What does it do?Edit
Using the Knapping or Pressure Flaking technique you can make tool heads or handles. The stonemasonry skill reduces the ammount of hits you need to convert stones and the quality of the stone also will get better!
Rising the skillEdit
What does it affect?Edit
The stonemasonry skill affects 2 things: The number of hits you need to convert something with a hammerstone and the quality of the item you get.