Barrel & Grit field manual
Best rocks for a rock tumbler: what to collect and what to leave out
Agate, jasper and many quartz-family stones are reliable starting points because they are hard enough to polish and can be grouped into compatible batches.
Start with Mohs hardness around seven
Agate, jasper, chalcedony, quartz and petrified wood commonly tumble well together. They resist rapid wear and can take a bright polish when the surface is sound.
Choose solid pieces
Look for dense rocks without deep cracks, crumbly seams or large open pits. A promising color cannot compensate for a structure that will trap grit or break apart.
Mix sizes, not hardness
A useful barrel contains larger pieces, medium stones and small fillers. Keep the hardness similar while varying the size so the load rolls smoothly.
Test unfamiliar material separately
If a steel nail easily scratches the rock or powder rubs from the surface, do not add it to a batch of agate. Run a small test batch before risking better material.