Choose a variety of rocks to add to the tumbler: hard igneous or metamorphic rocks (we chose basalt and granite, both igneous), as well as some softer sedimentary rocks (we chose sandstone and shale).
Lay the rocks on a sheet of blank paper and draw around them to show their original outlines: their size and shape. Label the shapes with the rock types.
Add the rocks to the tumbler, to fill it at least half full. Add some quartz hammer stones if more rocks are needed to fill up the tumbler.
Add a cup of water. Seal the tumbler barrel and start the tumbler.
Leave tumbling for an hour, then take out the rocks. Rinse them off if necessary and lay them on the paper of outlines to see how they have changed.
Some of the softer rocks (e.g. our mudstone) will already have started falling apart, and maybe even making mud. The harder rocks may be starting to smooth out on their jagged edges.
Run the tumbler for longer, a few days or a week, and then compare the rocks to their outlines again.
(If necessary, the tumbler may need some more water during this week, as some of the rocks turn to mud.)
The softer sedimentary rocks may have completely turned to mud after a few days - they have weathered away completely.
The harder rocks will still be close to their original size, but will be smoother.
This is the process of weathering - as rocks bump together, little bits get knocked off and the rocks are worn away.
The speed of weathering depends on how hard the rock is. Igneous and metamorphic rocks are much harder and more resistant to weathering than sedimentary rocks.
Some natural landscapes are fantastic shapes due to the uneven weathering of different kinds of rock.
