If you are confused, then I have failed in explaining things clearly. Sorry.
If you can turn the confusion into a couple of key questions that might clear things up, that might be the best way to clear things up.
Meanwhile I could take another shot at clearing things up:
I like what John said in another thread where ceramic came up:
What I like about ceramic media is it acts like very small rocks in the mix to speed up the process during all steps while acting as a cushion in later steps. All this while not picking up grit so can be washed out with the rock after each step and used in each step without fear of contamination. Small gravel, saw trimmings, etc. would be very good also. A friend was telling me recently about bags of white quartz at his local garden center for planters/flower pots. Not sure how small the quartz is but that could be a great, cheap filler for vibe or rotary tumblers and the tumbled quartz would be a nice plus.
it is all about abrasion, and there there are 3 issues (related to filler, more issues related to amount of water, amount of grit, speed of motor ...) to get good results: 1) size, 2)weight, 3) cleaning/sorting, and these issues play out a little differently in rotary and vibe tumblers.
Size:In a rotary rocks ideally slide down a slope of other rocks as the barrel turns. Not ride up to the top of the barrel with centrifugal force, not an avalanche where rocks roll and bounce down, but a slide - as if a tiny hand held it and ran it along a hillside of sandpaper.
You want rocks on path C to slide over lots of grit on surface B. The stuff down in A does not matter, and if your rocks launch onto trajectories D or E because of too fast a barrel or too empty a barrel, then there should be chipping.
In the vibe the rocks may go round or roll over or make figure 8's depending on the barrel design, but the work is actually done by very high frequency rubbing of the rocks against each other. Much like a hand-held reciprocating wood sander. There is some benefit to moving the unit back and forth or round and round, but the work is done by the rapid reciprocating of the sandpaper against the wood.
Now consider a bucket full of billiard balls. Each ball may be touching upto 6 or 7 adjacent balls, but that is not many, and the point of contact is very small compared to the whole surface of the ball. It would take a long time for these balls to work each other smooth if they had rough surfaces because the amount of surface doing the work at any moment is very small.
Now imagine the same bucket of billiard balls, but a bunch of small marbles, and scoops of steel BBs have been added. Each ball is now in contact with numerous marbles and steel BBs in addition to approximately 6 of the billiard balls. The amount of contact surface available to do the sanding work is much much greater, and the work will get done a lot more efficiently.
Weight:In a rotary it is easy to get too much weight and the belt might slip, or the motor might strain and burn out. In a vibe, too much weight may dampen the reciprocating action, but is less likely to burn out the motor. It is also harder to get too much weight unless you have really heavy rocks. These units have been designed to run with a full bin of average weight rock.
In a rotary the force that goes into sanding is supplied by gravity. As the rock slides down the gritty slope, it is pressed against the slope by gravity.
In a vibe the thing that governs the force that goes into sanding is the mass of the material. Imagine shaking a bucket of glass shards versus a bucket of Styrofoam peanuts. There will be some force in the collisions of the glass shards, but no matter how hard you shake, there will not be much force in the collisions of the Styrofoam peanuts.
Therefore, plastic is generally used in rotaries to achieve the benefits of small size without the cost of weight. Likewise, ceramic is used in vibes to get the benefit of small sizes without losing the necessity of mass.
Cleaning:Plastic will embed grit and should not be moved along with the rocks. You can easily separate them by floating them off the rocks in a bucket of water.
Ceramic will not embed grit, and in fact smooths and polishes, so you can move it along with the rocks. This is good because it has to be picked out by hand which can be a real pain. (Sometime sieves can help).
Little chips and small saw scraps can be used instead of ceramic, but often these have lots of pits, cracks, and crevasses. It is after all the waste material that was cut away, generally because of its flawed qualities. These flaws will embed grit and you have to clean really well in order to move them along with the rocks.
In my vibe units, I generally leave these small bits in the course until they are ground away to nothing. It helps save on the cost of ceramic. I just pick out my relatively larger rocks that I am interested in and move them along, adding ceramic if I need to.
Maybe once a year I polish up a bowl of the better looking small stuff. If it is plain or odd shaped or pitted, it just stays as fodder for the course grind. But If I can quickly sort out some colorful, rounded, smooth chips, these can be fun to polish.
I hope this long account isn't too tedious and that it makes the trade offs clear.
I will purchase some and, experiment with and with out it. Results will be posted here.
