It’s painful to go through your freshly tumbled rocks and hold your breath when you see a great one and then inspect for problems. Sadly, some of my best-looking pieces have suffered from undercutting during the rock tumbling process.
Rocks typically undercut and pit because they are made up of materials of different hardness. During the rock tumbling process, softer or more brittle sections can crack out before other sections.
Dallasite, the colourful jasper breccia I find on Vancouver Island, is made up of several different materials and so is prone to undercutting. Some pieces are better than others but it’s very sad to see a great piece marred by undercutting. Freckled Opal tends to pop out its vesicular opal bits too, as they are softer than the surrounding basalt.
When rocks undercut, you have two options: keep tumbling them, but make as absolutely sure as possible no residual pieces of grit are in those undercuts or pits, as the grit will come out in the next step and scratch your batch up; or throw them back to the coarse grind step.
I judge undercuts on a case-by-case basis. I am mostly concerned with having as smooth as possible rocks at the end. I seek zero imperfections but sometimes I can tell a rock is at its best likely state after coarse grind despite some small scars, so I make sure to get any grit out and move them along.
The crappy thing about putting rocks back in coarse grind is they lose more size. Some rocks get withered down majorly before being smooth and hard all over. Some are just not destined to become tumbled stones.
I can’t finish this post without a mention of Undercutters Pizza. Take care.