Smooth, shiny rocks are relatively common in the natural environment, as small stones roll in the surf or on stream bottoms for centuries. This constant abrasion against other rocks creates stones that are pleasing to both eye and touch. Humans have improved on nature when it comes to smoothing stones: by using a mechanical rock tumbler we can polish stones to a glossy finish that is so attractive that polished samples are often used for jewelry and other keepsakes.
Load the rock tumbler with the rock and mineral samples. Weigh the samples on the scale to avoid exceeding the tumbler's rated weight.
Add about one tablespoon of coarse (60-90) grit to the tumbler for each pound of rock. Add some gravel and sand with the grit if there are only one or two large samples. Add sufficient water to cover about two-thirds of the rocks.
Tumble with coarse grit constantly for two weeks. Check the tumbler after five to seven days to see if the grit has broken down. When the grit breaks down, pour off the water and add coarse grit and water as before. Continue tumbling for a total of two weeks.
Check the rocks for edges. Continue tumbling with coarse grit for another week if there are any sharp edges or corners.
Empty the contents of the tumbler. Wash the rocks to remove all traces of grit. Empty the tumbler and wash and rinse it entirely, then allow to dry. Examine the rocks, and discard any broken or very small pieces.
Return the rocks to the tumbler with medium (120-220) grit and water in the same amounts as in the coarse polishing step. Tumble for another week to 10 days.
Empty the tumbler, and wash the rocks thoroughly to remove any trace of the grit. Wash out all traces of grit from the tumbler, and dry the barrel. Discard any that are broken or undersized.
Return the rocks to the tumbler with pre-polish (500) grit and water as above. Use about three-quarters of the amount of grit as used in the first two tumble stages. Tumble for one week to 10 days.
Empty the tumbler, wash out any remaining grit, and dry the barrel. Wash the rocks thoroughly to remove the pre-polish grit. Inspect the stones and discard any are too small to polish (less than one-quarter inch or so) or that have broken.
Return the smoothed rock to the tumbler with polish-quality grit and water. Tumble for two weeks.
Empty the tumbler and wash both tumbler and rocks thoroughly. Return the rocks to the tumbler with water and a small amount of bar soap. Tumble for four to eight hours to clean off any remaining dirt or crushed grit .
Hard minerals such as agate or jasper produce a soft, glossy polish; but minerals such as calcite or fluorite that are not as hard can be smoothed but not polished. Be patient. Polishing each batch of rocks takes several weeks. Check on the tumbler from time to time and add water or grit if necessary.