Why Don't More Of Us Tummble Beautiful Granite ?

In my 2 1/2 years of tumbling I've done lots of granite because other than quartz and some fairly bland jasper that's all that there is to rockhound around here. Granite is appealing in the wild but it undercuts like crazy which to my shiny oriented eyes and brain is very distracting and it seems I'm not alone. I have two large trays on my coffee table full of my best tumbled rocks....one tray is shiny quartz and shiny brown, red and butterscotch jasper...the other tray tray is mostly granite with some shiny black basalt and a few shiny agates(not from around here). When people see the 2 trays they nearly always go for the first tray. It's an individual thing for sure but for me shiny beats undercutting any day no matter how shiny the rest of the granite is. I don't rockhound it any more unless there's some striking color or patterns going on. Having said all that I still have some granite in the tumbling queue so If you see some granite in near future photos it's not because I'm picking it up anymore...

I tumble both granite and petoskey. No blow outs (yet). I do not burp/check the tumblers daily.
The granite is self collected from my land. Some achieved a great, even shine. It all depends on the composition of the granite.

I have lots of granite counter scraps which I want to cut into tumble sizes. That will need to wait until summer.

Thanks for all the good info. Granite seems very common and colorful with great patterns. I'm wondering how to stop the undercutting, if possible.

Does granite round out quickly and if so would limiting stage 1 reduce the undercutting? I'm thinking it is in the Mohs 6-7 range.

I really like all the colors and patterns of granite, or what passes for it, at the local countertop store. They have a huge dumpster full of various granite scraps in the back of the store and said "Feel Free" when I asked them about taking a few pieces. Maybe I'll do some experimentation and see what I come up with.  I just bought a Thumler's UV-18 is on the way.  Between my TT model B and that I'll see what I can do with some nice granite.

I know you can put a spoon of baking soda into the mix it is supposed to stop the gas build up.

I have cabbed granite on my DP Titan and never had issues with it, but that is a much more controlled process.

Just having sections of quartz and felspar creates issues with granite.
Granite by definition is composed of 80% felspar and quartz.
The felspar is going to cut faster being softer than quartz and the tumbling process will leave an uneven surface often referred to as undercutting.

(Granite) gneiss is another animal, it usually tumbles better having gone thru a metamorphic process often resembles granite.
Granitic gneiss is seriously overlooked for a tumbling choice.
But it can be difficult to differentiate from granite. Wavy patterns and banding caused by metamorphosis are one telling hint that you have gneiss.
It is often found near granite sources.

"Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, dolomitic gneiss, etc."

Don't take it for granite...

I don't think the problem with consistently producing good, tumbled granite is due to the 80% quartz/feldspar. It's the remaining 20% of the rock's make-up that causes me headaches. Mica and hornblende are both softer than the feldspar and quartz producing undercutting in the stones.

I can get lots of nicely water-shaped granite stones right off the dirt roads in the Adirondacks. Some polish up nicely. Others undercut like crazy. I haven't done a lot of it yet but I did notice that trying to tumble all the way through in a rotary produced more undercutting than running them in a vibratory tumbler. I've got a bunch of stones that I'm going to start over with in a small barrel (3-4 lb) to see if I can even out the surface before switching them to a UV-3.

I wish you luck gatorflash1.
This is a tray of pink granite like stones. Collected from gravel along a railroad track.
Probably a mix of gneiss, granite and similar to granite stones. Just saying that I.D. can be tricky.
The pink color is about certainly pink felspar. I failed at polishing these. And had undercutting.
The biotite did not help matters.



If you are out collecting in a granite rich location you will likely find some pure pink or white felspar.
You might consider tossing some pure felspar in with your granite load and see how it tumbles since granite is composed of felspar.
Felspar can be a tricky tumble.
Felspar is Mohs 6 and quartz is Mohs 7. Knoop hardness is another useful hardness number the quantifies hardness.
Felspar is quite a bit softer than quartz when looked in terms of Knoop numbers, actually closer to glass(530) at 560 verses quartz at 820.
Knoop 560 is normally a tricky-to-polish tumble.
Note aluminum oxide(alumina) is Knoops 2100 and silicon carbide is Knoop 2480 and silicon carbide is far more aggressive than alum ox.
Garnet at 1360 will polish quartz but it takes a long time.

Thanks to all for the good info. I have cabbed granite with excellent grinding and polishing results, but using a cab machine is a much more controlled and faster process than tumbling.  Granite and related comes in just about every color of the rainbow, great patterns,  and is very low in cost, mostly free,  which appeals to my Scotch ancestry.  The counter shops in my area, which is not agate rich, seem to have tons of scrap behind the shops or in the dumpsters which is free for the taking as it is otherwise headed for the local dump.

The granites I polished were very fine grained which is probably the secret to getting good tumbled granite.


James it looks like you have a good supply of rough to work with.  I wish I lived closer as I would try to polish it with the diamond lap on my cab machine.