Pitted and white

Ha. Well, I'm hardly a pro either but have 2 cents to offer. It sounds like you've definitely been paying attention. Agreed that pix would help.

First off, about granite. Although different types of granite have different materials that make it up, they can be a tough one to tumble for that very reason. It's not uncommon to have softer materials mixed in with harder material...therefore making it quite easy for some material to wear down faster than others. (e.g. pitting, also known as under cutting) Under cutting can happen in any stage, but it seems to me stage 2 & 3 are more prone due to the grit's lesser ability to cut through the harder material. Although granite can and will take a shine (e.g. countertops), you have to realize that this is polished using a much different method from tumbling. Give those rocks a real close look - with a loupe, if you can - is it the same material that's tending to wear away more quickly?

Onyx, from what I've read, comes in a few flavors. There's the "real" stuff that's about MOHS 7...and other types that are much, much softer. I'm not sure which you have, but if you can't scratch it with a knife, chances are good you've got the "real" stuff. Softer materials definitely tend to pit more than others and they usually have to be handled differently.

Something you don't mention is how much plastic media you're using. I find it best to measure out specific quantities, rather than going with the "to the 2/3 mark" rule. I've found some rocks require a lot of plastic media - particularly those that are more brittle - sometimes upwards of 25-35% of the barrel. I usually start around 1TB-2TB per pound of rocks and work up from there, if it's needed. If I've previously had problems with a particular type of rock, I'll double or triple that the amount.

Another issue that could be here is over all rock size. If you have several large rocks, they tend to crash pretty hard - especially is these larger barrels. I tumbled for years in a 3lb model 3A...and I saw some pretty big differences in the 6lb. It's important to have a good mixture of sizes - perhaps 1/4 to 1/3 of of small rocks - to help cushion the fall of the bigger ones. There should be a good mixture of sizes...and only maybe 2 or 3 larger stones (2"ish), with the rest being smaller, in the 6lb barrel. This typically causes fracturing/cleaving, but it's possible you could be getting pitting due to the type of stone.

Lastly, can you describe your slurry? It should be fairly thick - many have described it as pancake batter consistency. Part of the slurry's job is to help slow the tumble by providing viscosity to the rolling rocks. I've found it a real pain to get consistent slurries in the 6lb barrels...much more so than 3lb barrels. I tend to use a fair amount less water - usually just over an inch under the top of the rocks. I've also just started playing with 1.5TB of grit per pound of rock with 1/2" under the top of the rocks. I've noticed that the higher density of grit is helping create a more consistent, thicker slurry.

Another point - you fairly well accurately describe your rock shapes. It's possible that some harder/more pointed edges could be causing damage. I've noticed this in a more pronounced way when I've tried to mix freshly broken rocks with previously course tumbled rocks in the course stage. If other things don't help, you might try a batch that's been rounded down more in course.

That's what I can think of at the moment. Perhaps someone with onyx experience might enlighten us with a recipe that's worked. You might also do some searching in that regard.

OK, Onyx as used in counter tops is usually a form of travertine and is pretty soft for tumbling, usually less than Mohs 5. Plus a lot of travertine is mixed hardness which adds another difficulty factor as some areas are softer than others and may undercut even when done correctly. If I were to try something that soft in the rotary, I'd start the grind with 110/220 as that should round the stone pretty well. Then I'd follow with 500, tripoli and your polish of choice with tons of plastic pellets to cushion each stage. All that and I'd still be surprised if you got a decent shine due to the softness. Probably a satin finish would be as good as you'd get. Granite same deal. Some is real homogeneous but lots is mixed in composition and hardness. Most have feldspar which is relatively soft and then maybe even garnet or quartz which is harder so again you have the undercutting which shows up as those pits and irregularities....Mel