You open the barrel and find stones with chipped edges, white bruise marks or cracks running through them. It’s frustrating, but it’s also one of the easiest tumbling problems to fix. Almost every case comes down to how the barrel is loaded.
What you’re seeing
- Chips: small pieces missing from edges and corners, usually from stones knocking hard against each other.
- Bruises: white, cloudy crescents or star-shaped marks just under the surface of agate and quartz. They are tiny fractures caused by impact.
- Cracks and breaks: a stone that splits along a weak line, often one that was already there when it went in.
The most common causes
- A barrel that’s too empty: below about two-thirds full, stones fall and crash instead of rolling.
- No cushioning in the finer steps: smooth stones need plastic pellets in pre-polish and polish.
- Mixed hardness: harder stones batter softer ones. See which rocks can you tumble together.
- Brittle stones: obsidian and glass chip easily even in a well-loaded barrel.
- Cracks that were already there: tumbling finds every weak line in a stone.
- Stones that are too big for the barrel: a few heavy pieces hammer everything else.
- A thin slurry: too much water or too little grit leaves the stones nothing to cushion them. Keep the water just below the top layer, and use about 2 tablespoons of grit per pound.
How to fix a damaged batch
- Pull out broken stones. Their sharp edges will scratch everything else.
- Go back a step. Bruises and chips have to be ground out. Small marks usually clear in the medium/fine step. Deep bruises need another week in coarse grit.
- Reload the barrel properly: two-thirds to three-quarters full, topped up with filler.
- Carry on through the steps with fresh grit, and check dry stones before moving on.
How to prevent it
- Fill the barrel two-thirds to three-quarters full every time.
- Add plastic pellets in the pre-polish and polish steps, and in every step for brittle stones.
- Sort stones by hardness, and run soft and brittle stones on their own.
- Check stones for cracks before loading, and between every step.
- Mix stone sizes so small ones fill the gaps between large ones.
For more fixes, see the common problems section of our four-step rotary guide, or send us a photo and we’ll help you work out what went wrong.