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If you've ever been to the beach, you've probably picked up rocks that are
perfectly smooth and rounded. The ocean is a natural rock tumbler: as
rocks are rolled by the waves and rubbed against sand and each other, their
rough edges and corners are filed away. A rock tumbler can mimic this process
and even take it a step further by adding polish to make the stones shine. Rock
tumblers work much faster (4-6 weeks) than the ocean because their motion is constant and
they use an abrasive substance called silicon carbide, which is much harder than
Lapidary is the art of cutting and polishing gemstones. Rocks and
minerals come in all different colors and patterns, and polishing them to a
smooth shininess brings out their incredible beauty. Polished rocks are often
used in jewelry.
When selecting rocks to polish in a tumbler, there are several things to
- Size: You should include a mix of sizes in your tumbler, from
1/4" to 1". The smaller stones rub up against all the contours of the bigger
stones, ensuring a uniform shaping.
- Hardness: All the rocks in a batch should be around the same level
on the Mohs hardness scale. The scale runs from 1 (talc) to 10 (diamond)
and at each level a mineral can be scratched by something of the same or higher
level, but nothing lower. If you tumble rocks of different hardness, the softer
ones will be worn down too much and the harder ones not enough. If your rocks
scratch each other equally, they are around the same hardness. The ideal
hardness for tumbling is between 5-7. You can test the hardness level of your
rocks using a set of
Mohs minerals, or by trying to scratch them with a pocket knife (hardness
5.5) or a steel file (6.5). Harder rocks will take longer to tumble than softer
- Surface shape: Try to tumble rocks with similar surfaces. Rocks
that already have rounded edges will need less time in the tumbler than
rocks with sharp edges, so you should tumble them in different batches.
- Beauty: Pick rocks that have colors and patterns that you like,
especially when wet. (Polished rocks look similar to what unpolished rocks
look like when wet.)
Your rock tumbler will come with full instructions, but here are the basic
steps to polishing a set of rocks:
- Fill the tumbler barrel 1/2-3/4 of the way full of rocks. The rocks lose
a lot of their mass in the tumbling process, so if you start out with too
few they won't tumble properly later on. If you have the barrel too full,
the rocks won't be able to move enough to be shaped properly.
- Add the coarse grit and enough water to come up just to the bottom of
your top layer of rocks. Together the grit, water, and rock particles that
are worn away are called a "slurry."
- Make sure the barrel opening and lid is clean and dry so it seals
properly, then begin tumbling. Carefully open the barrel after about 24
hours to release any gas that has built up inside and to check that the
slurry is forming. The slurry will look
like a creamy dark gray mixture.
- Let the rocks tumble in the coarse grit for 6-7 days, checking
periodically. After a week, remove the rocks and thoroughly clean them and
the barrel of all grit. Warning: do not rinse the slurry down any
household drain! It will harden like concrete in your pipes. Put it in a
disposable container and throw it away.
- Sort through your batch of rocks and remove any that have cracks, chips,
or pits. If they have any cavities where grit can get stuck, they will
contaminate the future steps.
- Repeat these steps with the remaining stages of grit/polish, cleaning
the rocks and barrel between each step. (For harder stones, you may need to
repeat the coarse grit step if they aren't shaped by the end of the first
week. You need to repeat with new grit, because the grit loses its
abrasiveness after a week.)
- By the end of 4-6 weeks you will have smooth, shiny, and beautiful
- You can display them in a display case or vase, or use jewelry wire to
make pendants and other jewelry.