What Rock Are You Tumbling?
People who operate rock tumblers are always interested in tumbling new types of rocks. They love the surprise of opening the barrel
at the end of the polishing step to see what they have created. Some types of rock work great in the rock tumbler, while others are
a waste of time and supplies.
Most Frequently Tumbled Rocks
The most frequently tumbled rocks are agates, jaspers and several varieties of quartz such as amethyst, citrine, aventurine,
smoky quartz and tiger’s eye. Other favorites are petrified wood, obsidian and a few interesting feldspars such as amazonite,
moonstone, sunstone and labradorite. May other types of rock can be tumbled but they often require special types of polish or a
very experienced person.
Characteristics of Good Tumbling Rocks
1) They are very hard. Soft rocks can be very difficult to polish in a tumbler but those which have a hardness
between about five and seven on the Mohs Hardness Scale work the best. Rocks harder than seven are so hard that the abrasives
cut them slowly and they often do not yield a good polish in a tumbler.
2) They are dense, without pore spaces or fractures. Pore spaces trap particles of grit and carry them from one
step of the tumbling process to another. This contamination of coarse, medium or fine particles in the polishing step will ruin your polish. There’s nothing worse than having a great polish but your rock is streaked with coarse grit scratches.
Fractures are another problem. They cause the rock to break up in the tumbler and the sharp edges on the broken pieces will
scratch other rocks. If you have a rock with lots of fractures break it into smaller pieces that are not fractured. If you
have a rock with lots of pore spaces it might be best to throw it away.
3) They have a smooth texture. Good tumbling rocks break to form smooth surfaces that are not “gritty” or "grainy." Rubbing the rocks
together does not dislodge particles. Rocks that produce debris when rubbed together will not polish and they will
produce “grits” that will scratch up every other rock in the barrel.
Agate is the most popular rock for tumbling. It is a translucent material that occurs in a spectrum of colors. Pieces of
agate often have natural decorations of bands, eyes, clouds, plumes or mossy formations. Agate's translucence often gives it an internal glow
that enhances its beauty. Combine these characteristics with an ability to accept a very bright polish and you will understand
why agate is so popular.
Agate is a microcrystalline variety of quartz that generally forms when ground water deposits minerals in rock cavities. Air pockets in basalts,
rhyolites, andesites, limestones, and shales can be a place where agate can form. It also forms in fractures
and bedding plane separations.
Agate is almost always much harder and more resistant to weathering than the rocks in which it is enclosed.
When those rocks weather away agate nodules will be left behind. These nodules are frequently found in stream beds and
dry washes. In some locations they can be picked up in abundance!
The bands, eyes and colorful patterns that occur in agate are a result of its crystallization history in the rock
cavity and the types of impurities carried in by ground waters.
Jasper is very popular as a tumbling material. It occurs in a variety of colors but usually it occurs in
white, shades of gray, and earth tones such as red, yellow, orange and brown. Occasionally other colors such
as green jasper are found. Jasper's color patterns include variegations, plumes, swirls, brecciation and more.
It generally accepts a bright polish and is a durable tumbling material.
Jasper is another variety of microcrystalline quartz. However, instead of being translucent it is opaque to
semi-translucent. It occurs as a variety of fracture fillings, mineralizations and replacements.
Jasper, chert and
flint are very similar materials. They are all opaque varieties of cryptocrystalline quartz and there are no precise
definitions that separate them.
Quartz is the most abundant mineral in Earth's crust and it often occurs in forms that meet the "characteristics of
a good tumbling rough" listed above. Quartz is transparent to translucent and occurs in a wide range of colors. Amethyst
quartz, clear quartz, white quartz, rose quartz and smoky quartz are very popular for rock tumbling.
Aventurine is another variety of quartz that is popular with rock tumblers and jewelry makers. It is a quartz that
contains inclusions of mica, iron oxide or other minerals. These inclusions have a preferential orientation that produces
a glittering appearance when the stone is viewed from the proper angle.
Tiger's eye is a popular tumbling material that forms when crocidolite is replaced by quartz. This produces a stone with
a fibrous texture that interacts with incident light to produce a silky luster known as "chatoyancy."
Petrified wood is formed when woody material is buried in an ash fall or other sedimentary deposit and then replaced by
siliceous or carbonate materials delivered by ground water. It often preserves a view of cell walls, bark, growth rings
and other plant structures.
Petrified wood is a very popular tumbling material. Although it is not especially abundant, it can be colorful and the
novelty of producing polished stones that exhibit preserved plant structures make it very appealing to many people.
Obsidian is a volcanic rock that forms when molten rock material cools at a very rapid rate. The cooling is so rapid that
the molten rock solidifies into an amorphous glass instead of crystallizing into organized mineral structures. This type of
cooling often occurs on the margin of a lava flow.
In the rough, obsidian looks like glass cullet or glass slag. It shares many properties with glass, including its
conchoidal fracture, hardness of 5-6, vitreous luster and low toughness. It is a challenging material to tumble because
it tends to chip and bruise. However, careful work and the use of ceramic media or plastic pellets for cushioning will
allow you to produce a beautiful product.
Most specimens of obsidian are a lustrous black color, however, brown, red and other colors have been found. Some specimens
contain numerous white spherulites of cristobalite which give it a black-and-white spotted appearance known as "snowflake
obsidian." Other specimens have swirls of brown and black glass which are known as "mahogany obsidian." "Apache tears" are
round nodules of obsidian that weather out of perlite.
A few feldspars are popular as tumbled stones. These include amazonite, moonstone, labradorite and sunstone. They
can be challenging to tumble because they have planes of cleavage which will cause the stone to break if it gets
roughly tossed about in the tumbler barrel. The chance of breakage can be reduced by filling the barrel to the
proper level and cushioning with plastic pellets or small ceramic media.
Amazonite is a green microcline feldspar named for the Amazon River, although the material is not known to occur in
that area. It occurs in a variety of shades of green ranging from pale green through deep green and blue green.
Moonstone is an orthoclase or an albite feldspar with a pearly to opalescent luster and a play of light
known as adularescence. It occurs in a range of whites, pinks, grays, greens, oranges and browns.
Labradorite is a plagioclase feldspar that exhibits a phenomenon known as labradorescence. This is a play of
colors (usually electric blues, greens and yellows) that can be seen when the stone is moved under incident light.
Sunstone is a plagioclase feldspar that contains tiny flakes of copper as inclusions. These copper flakes have a
preferential orientation that causes them to reflect light simultaneously when the stone is moved under incident
light. This aventurescence produces a copper-colored flash from which the name sunstone originated.
Other Popular Tumbling Rocks
Many other types of stones are tumbled but usually not with the frequency of the varieties listed above.
Hematite, lapis lazuli, lepidolite, malachite, mookaite, prehnite, rhodonite, sodalite, and zoisite are
occasionally seen as tumbled stones.
Howlite is commonly tumbled, however, its white color makes it uninteresting to many people. However, it is frequently
dyed to resemble turquoise.
We hope that you have a chance to tumble many varieties of stone!
| Here's a great collection of tumbled agates. Agate is the most popular tumbling rough because it often has great color, interesting patterns (bands, eyes, moss, lace, fortifications), translucence, and it usually accepts an exceptionally bright polish. It is usually easy to polish and is a great tumbling rough for the beginner.
|Jasper is another very popular tumbling rock. It is a semi-translucent to opaque material. It can be any color but is most often reds, earth tones and green. Most jaspers accept a very bright polish and polish easily for the beginner.
|Petrified wood is a favorite with people who do rock tumbling. The pieces of petrified wood above show distinct wood grain patterns.
|There are many varieties of obsidian that are popular with people who tumble rocks. The rough specimens above on the left are mahogany obsidian (named for its brown wood-grain swirls). The two center obsidian specimens are rough Apache tears. The specimens at right are snowflake obsidian (named after the white cristobalite blooms).
|Blue is an unusual color for a rock and that makes materials like sodalite, lapis lazuli and blue chalcedony especially popular with people who tumble rocks.
|Agates with "eyes" are rare but if you tumble enough agate you will probably accumulate a nice collection of tumbled stones with "eyes."
|Some people get tired of tumbling rocks all of the time and decide to try tumbling something familiar like making tumbled glass from bottles or landscape glass. It is very popular for use in crafts and as a ground cover in potted plants and aquariums.