How much of a stone is worn away during any stage of the tumbling process is of course dependent on a number of factors. The three major ones are the hardness of the stone, the size of the grit/abrasive used, and the length of tumbling time.
Most of the books on stone tumbling don’t comment on this issue, though Del Fairfield in “Lapidary” (a “Teach Yourself Book” published by St Paul’s House, London, in 1973) mentions that seven days tumbling in grade 80 grit leads to “about a 5 per cent reduction” in weight (page 15).
I recently decided to pay some attention to this question so I purchased a small set of digital scales. I have done the calculations for two batches of stones so far.
For a batch of Riverton Back Beach stones that went into a 4lb barrel and were tumbled for 6 3/4 days with 7 tablespoons of 100 grit: Starting weight of stones = 1620 grams; End weight of stones = 1424 grams. This means that 196 grams of the stones were worn away from them, which is just over 12% of their weight.
For a batch of Birdlings Flat stones that went into a 3lb barrel and were tumbled for 5 1/2 days (there was a loss of electricity which shortened the normal one week’s tumbling time) with 4 tablespoons of 220 grit: Starting weight of stones = 862 grams; End weight of stones = 820 grams. This means that 42 grams of the stones were worn away from them, which is just under 5% of their weight.
