By
Alan Silverstein,
ajs@frii.com.
Last update (this page): Apr 3, 2026
These questions are collected from various sources and are in no particular order. The questions are mostly from other people, and the answers are mostly mine.
Contents of this section:
Yes indeed it does. Also it usually has fewer inclusions or holes than many other agates. But not always. Some of the moss agate found in the Pawnee National Grasslands of northeast Colorado can be rather bubbly.
Pellets are not needed for agates, only for stuff that frosts. Meanwhile, if the agates still fracture, it means either the barrel is too empty, or more likely that they were just fractured and the coarse grinding didn't break them yet, but eventually they went to pieces under the continued gentle persuasion of the tumbler. I do occasionally see this too, I just shrug.
My tire tumbler is so rough it tends to weed out the weaklings for me now. But then again, I literally screen the washed stones with a 1/4 mesh to separate out the gravel toward the end of sorting, and I get 1/4-1/2 cup of gravel for each load. I do save it and sometimes just polish it, looks nice in glass jars.
I don't buy much of anything, I find my own, but I seldom bust anything with a hammer. Maybe if it's shaped weird I'll knock an end off or something first. But hammering creates cracks, so do it carefully. Too many rough stones break into smaller pieces anyway along existing cracks during rough grinding. I've seen videos showing people "preparing" agates for tumbling by smashing them with a hammer (loose or in a cloth sack), and I wince. Use a chisel, aim carefully, or get a bigger tumbler (grin).
I've bought broken (dyed) Brazil agate slabs by the pound and tumbled them to refinish them. This works great, and even these are seldom too big to go whole into my 12 lb Lortone rotary tumbler.
Someone else observed that usually slabs are created, sold, and bought to use for making cabochons (cabs). However, as long as they are not too big they are legitimate tumbling materials. Polished slabs or other relatively flat tumbled stones can be wrapped with copper foil and used to create "stained glass" panels or free-hanging window decorations that are very pretty.
Sure, mostly. I've polished slabs OK. The large flat sides can remain less than glossy longer than most rocks, so longer polishing time helps. But in a 3-lb unit, you aren't talking very big pieces anyway. I'd just try it.
Now, I do not put slabs in my truck tire tumbler any more unless I want them to break up into smaller pieces... It's just too rough. I put them in the 12 lb unit for coarse grinding -- although I try to minimize coarse grinding in the little barrels, to extend their life, I use the truck tire for most of it.
The sides are saw-smoothed, but the edges are rough. If you don't coarse-grind, you'll still have some relatively rough edges, that's all. Up to you.
Many of my tumble-polished slabs with nice rounded edges (from starting all the way with coarse grit), I wrap with copper foil for making window decorations using stained glass technique... Even rough slabs (already polished in Brazil) work, so long as the edges aren't too rough to hold the foil.
There really doesn't seem to be much consistency here. The lapidary catalogs sell a lot of different powders by a mix of generic names like cerium oxide, brand names like Linde A, or just "prepolish", and it's always confusing. Plus, the same compound can apparently be used either way depending on the grain size! The only consistent fact seems to be that all polish powders other than diamond (typically not used in tumblers due to cost) is some sort of metal oxide.
I've settled on cerium or tin oxide for final polish because it works well, even a mix (when I refresh old reused polish slurry by adding more powder to it). I've learned to avoid sapphire / alumina / aluminum oxide / Linde, or anything like "rouge" or "iron", because they don't seem to work well. Offhand though, I can't recall what I currently have at home for "prepolish"; I bought 5 lbs of something long ago. It's reddish, so it might actually be iron oxide (rouge); all I know is I'm getting acceptable results from that (reusing the slurry) followed by cerium/tin slurry. I probably just bought something labeled or described as "a good tumbling pre-polish" and don't know what it is.
You can try going directly from fine (silicon carbide) to polish (cerium/tin) and run on it twice as long. I finally did this experiment, though, and wasn't pleased with the results. The rocks just weren't shiny enough for me.
Also, ironically, my pinkish cerium oxide that I use for polishing is not "optical grade" -- much more expensive, I just bought a couple ounces for $5 and even that is not pure white. One catalog lists "French cerium oxide" at $7 for 2 oz.
After unscrewing the "knurled knob" (black plastic) and taking off the aluminum washer, I use the round end of a bottle opener between the metal and the rubber to pop off the metal lid -- carefully, so as not to bend it, opening it a little at a time and working my way around. I think they recommend using the big washer for this, but that chews up the edge. Over time the bottle opener also mars the edge of the lid, so it cuts into the rubber roller a little while spinning, but it doesn't seem to hurt anything.
Once the metal lid is off, I push out gently around the edge of the barrel while pulling up the inner rubber-covered disk by the center screw.
I ground off the opposite point of the bottle opener so it wouldn't bite my hand while using it. (grin)
I find it's hard to get glass to take a good shine, it's too soft and "frosts", even when I ran it with pellets. However, beach glass is frosted too (shrug).
I treat glass just like rocks except maybe I wouldn't mix them, just run pure glass. Also it's the only material I've ever seen outgas and expand rather than suck in the sides of the barrel -- watch for blowing the lid off, you might have to "burp" it every day or two.
I have simply broken up colored bottles and started coarse grinding them, with OK results, except no glossy polish, even using plastic pellets. It's like obsidian I guess. Being it's soft, probably you should use less grit and run for less than a week on coarse grinding.
No, cabbing refers to creating a "cabochon", which is a hand-polished stone with rounded surfaces, usually for jewelry, all the way from cutting a rough cab out of a slab to final polishing. "To cab" is an instance of English both abbreviating a longer word and verbing a noun.
You might be thinking of "cobbing", which I believe is a seldom-used word for smoothing a rough stone by chipping off parts:
In 1997 Mark Liccini wrote (and I edited),
There is different media you can use to cushion the stones and carry the polishing agent. If you want your cabs to stay in calibration, you will need to shape and one sand first. So you are really just saving the finer sanding steps and polishing. But on hundreds to thousands of cabs that is a lot.
Because you are finer sanding and polishing only, you would want to run the tumbler on a low speed anyway. This and the media, pieces of leather, walnut shells, etc will stop the breakage.
Give you one further tip. Tumble-polished stones can be identified by a small, poorly polished circle dead center on the bottom. If you want to avoid this, give a quick sand to the bottom too before tumbling.
In 1998 Paul Boni wrote (and I edited),
A. No soft stones, most turquoise, opal and etc. will get too "beat-up" (maybe someone else has a technique for soft stuff).
B. Any heterogeneous materials of varying hardness will undercut. This includes agates such as "dry head" which have softer layers.
C. Cleavable minerals will cleave in the tumbler. Topaz, Charoite, Sodalite, and etc.
Most agates work great. I've also had good results with, quartz crystal (rutilated, amethyst, clear, and smokey), garnet, and colored glass. In an experiment with Mexican crazy lace, I found that by using the tumbler I could produce calibrated cabs with no more than 10-15 minutes total time in each cab (beginning with pre-slabbed material).
The method: I rough the stones on a coarse 6" diamond grinding wheel. Only the small ones were dopped. The stones are then sanded on a fresh 220 grit silicon carbide belt on an expandable drum. I run my expandable drum slower than most folks so I can use the "give" of the rubber to wrap around the stone and reduce the ridges produced. (I can't get this part to work with diamond.) If done right (only a little practice is needed) agates finish this step with no ridges whatsoever. I then turn the stone over and kiss the sharp edge from the back of the stone. I should also note that I cut most of my cabs with a bevel.
The stones go into a Vibrasonic tumbler (manufacturer doesn't matter) with 600 grit and only enough water to allow the grit to cling to the stones. This is important! Most people use too much water.
I load the tumbler with the cabs and enough scrap (of the same material) to the weight specified by the manufacturer. Then I add a heaping teaspoon of grit (I use a 4 lb. tumbler). Then I turn it on dry and slowly add water until two things happen: 1) I see the grit clinging to the stones as they cycle to the top of the heap, and 2) the action of the stones becomes less violent and the heap rolls over on itself with a steady action. At this point the sound will be much more subdued as well.
I run for 12 hours, clean and rinse the load and repeat. After 24 hours, I inspect the load. This is usually enough. If needed the load can be run again. The load is then cleaned, prepolished, and polished. I get a high polish, minimal size reduction (can't measure any) and only a slight rounding at the dome, bezel edge. Too coarse a grit or too long in the grit will produce more rounding of the edges, deformations, and size reduction.
The important thing is to experiment with your tumbler and the material you are cutting. Keep notes! And modify your technique to fit the occasion. No one cookbook technique or formula will work for all cases and situations. Be patient and when you hit on a method that works for you, write it down, remember it, and (share it).
In 1998 Anthony L. Lloyd-Rees wrote (and I edited),
...hopefully you have a small barrelled tumbler because you won't want to share your cabs with any other stones and you will need enough preforms to make a tumbler load.
Second, all the same size helps.
Third, all the same material helps a lot.
Fourth, glue the stones of the same size back to back. This stops that little dent in the back of the stone from happening and also keeps a crisp girdle line. Also this practice preserves calibration and prevents chipping if you are using a rotary tumbler [which he disdains].
Note: The more care you take in accurately shaping preforms the better looking cabochons you end up with.
The grit size you start with is determined by how much stone you estimate needs removing, if larger than the finest is needed the shape will suffer. The cutting stage should be kept as short as possible anyway. Use your usual polishing techniques and compounds for the material you are tumbling.
I have only tumble finished stones cut with automatic cutting equipment. A tumble polished stone cannot withstand a 10x loupe. at least mine didn't. I feel personally that automatic preforming is the only excuse for tumble finishing.
I have never tried this with hand cut stones, I couldn't imagine anything worse than putting the effort into getting a nicely shaped stone and then missing out on the good part where you make it come alive in your fingers. I always thought this was the magic part, the moment the rock turns into a gem...
In 1998 Ted Robles wrote (and I edited),
Tumble polishing cabs. Yes, it can be done. I have done it, with good success, using tin oxide in honey with 4 mm glass beads as carrier. The thing to remember is not to cut the edges to razor sharpness. They need to be about 1 mm thick to withstand the occasional edge-to-edge impact. (I am assuming cabs of jasper, agate, or similar hardness.) Don't crowd them. About 2 volumes of beads to 1 volume of cabs is about right. (In a conventional tumbler it takes a week to 10 days.)
Apparently so! But the one website I found talking about it -- is long gone.
In general, yes, depending on how round and smooth you want it.
In particular some petrified wood has a "bleached rind" that's lighter than the interior. Often I stop grinding those pieces while the rind is still present, for a nice two-tone effect in the finished piece, even if it retains rough edges.
I've left my tumblers off for as long as 11 days I think, at any point in the weekly cycle. I've had no problems firing them back up when I return. I'm sure the slurry settles inside, but it doesn't harden like concrete so long as it stays wet, and the barrels turn over just fine after I flip the power back on. Note well that the barrels remain sealed.
Furthermore, given how the slurry does settle out, I'm wary of letting it run down any drains (at least I minimize it). It's not that you couldn't wash it down with a hose, at least if it's never dried out, but as someone else said (in email), how do you get the water pressure to the clog in your pipe?
This is something I've fantasized about but never tried. Cement mixers are not really cheap, like $180 and up for small ones new. Moreover I suspect the rocks and grit would chew up the insides in a hurry, they are probably bare metal and not designed for continuous use. You'd need to line them with spray-on truck bed liner or something first?
Also, any tumbler needs protection against rocks and slurry escaping -- probably not an issue with a cement mixer -- and against drying out -- you could probably cover the opening with a plastic sheet held on by a bungee cord or something.
If you're reading this and you've actually tried using a cement mixer as a rock tumbler, please let me know.
After I wrote that, I got the following from Susan Belyea, who said,
I have been using a cement mixer with a plastic barrel (steel is too noisy) to tumble glass cullet (waste product from my glass-blowing production operation) and it works fine. Splashes a bit -- I don't try to seal the mouth of it, just a hang a plastic sheet close-by to deflect the splashing. It takes 12-15 hours of tumbling 20 lbs of small chunks of glass with fine grit silica sand (which I use because I have tons of it around the shop; silicon carbide might be better) and a few gallons of water to get a nice frosted finish without obvious chipping on the edges of the glass.
Regarding the wear and tear on the plastic (cement mixer) barrel; it really depends on the model of barrel. The simplest barrels -- no undercuts in the mold making process with steel fins -- lasts just fine. There is a model called The Big Cat available at hardware stores all over which is not good for this purpose. I cracked a barrel twice with this model -- seems like there are too many places for glass to get stuck, and while I don't completely understand it, I switched to a different model and have no trouble.
I am only processing about 20lbs of glass in a pretty standard size cement mixer. I use only enough water to get a loose slurry going. I suspect if I tried to process larger loads, I'd get more splashing.
When I was building the tire tumbler, I did some math on the physics of the subject (which I still need to add to this website). I had to figure out what seemed like a good wall speed, hence rotation rate, hence motor to pulley to drive shaft ratio. (The 5/8" drive shaft is tiny compared with the 31" tire, so it squeeks like crazy if the tire is overloaded, and it can even give up completely with the tire stopping in place.)
Anyway, the tire takes about 8 seconds per rotation, which means it runs at about 0.7 MPH. (First number from memory, second number calculated based on a 31-inch tire, although I started with a 33-inch tire originally, but I can't find those any more.) This seems to be a pretty good speed. It does an effective job of fast grinding, although it busts up some rocks that are already cracked (maybe this is a good thing) and it leaves some crescent-shaped impact scars in larger stones (we're talking baseball-sized here!) that remain after polishing.
When the first tire wore out years ago, I estimated how many hours it had run, and from that figured I'd gotten about 450 miles out of it. (Also from memory.) Just goes to show that tire tread is a lot better at handling rocks on the outside than on the inside. (grin)
Anyway, now you can see why you can't put rocks in your truck tires and drive around -- unless you do it really slowly -- and then you'll wear them out really fast anyway. If you drive much faster, centrifugal force pins the rocks to the inside of the tread!
Uh, that's an embarrassing question. (grin)
Well the short answer is: I give most of them away, and ultimately I'm just borrowing the rest from my descendents and everyone else on the planet. I'm pretty sure that in a million years someone will find one of my polished rocks somewhere, they'll have no idea who or how it was produced, but they'll recognize it as clear evidence of intelligent manufacture.
Tumble-polishing rocks is fun for the sake of the process itself -- traveling, rock-hunting and collecting, cleaning and sorting, grinding and sorting (including a certain amount of "playing in the mud"), polishing and sorting. The result is 10-15 pounds per month of polished stones ranging from junk to treasures, although even my "treasures" are generally not on par with the commercially available mass-produced stones you can buy at nature stores, etc. Nonetheless I enjoy the hobby and have not noticed a decline in my interest level.
When I dump a polished load of rocks I sort it into "keepers" and "give-aways" while listening to radio or in front of the TV. The "keepers" add up for a while in flats, then I sort them again into "keepers" and "give-aways", and the "keepers" further by type into other flats, which I shelve. This does seem kind of silly, hiding them away, but there's a limit to how many rocks can be out on display in plain sight! Certainly the more special (to me) rocks end up in various places around my house. I find it handy to put them on or in paper or plastic bowls or trays so it's easy to move them for dusting, etc.
As for the give-aways, there seems to be an endless demand for free polished rocks. I take them to monthly Fort Collins Rockhounds Club meetings for other people to pick over. I offer them to anyone who visits my house, including relatives from far away. You'd be surprised how many people go from, "nah, thanks," to "OK, well just a few." At Halloween I set up a few flats under a light just inside the door and invite the kids, "two pieces of candy and as many rocks as you want to take!"
Finally, one more observation. "The more of anything, the less the value of every such thing." I've noticed that over time my previous "keepers" are easier to part with and often become "give-aways". There's a continual winnowing or high-grading process that's enjoyable, although time-consuming.