Tumbling by Mohs Scale

Boy, you are just full of good questions Julie!

I'm sure that other's will chime in with their various takes, but allow me to share mine-

Essentially, anything softer than about 3.5-4.0 probably isn't worth tumbling, certainly not in course as 9+ hardness grit any bigger than 120-220 will destroy such rocks in no time.

Mohs 4-5 material has potential, provided its somewhat homogeneous, but often things that grade in this range will have zones of varying harness which will be prone to "undercutting". In any case, this material can start in course but should be monitored every couple of days as it might cut a lot down faster than the week (or five) that harder material will teach you to expect.

Another option is to start with 120-220, perhaps run it longer than just one week, but at least you have less chance of losing too much of your material too fast this way.

And you might also want to be a little stricter in terms of what you move on, some softer materials often just aren't going to polish no matter what, so if something starts to look extra grainy or a little more porous at a pre-polish stage than it did before, might as well give up on that piece, as it isn't going to get any better going farther.

An example of how you might do it;
I often have obsidian pieces laying around that I might toss in to help fill a load of agates or jaspers, but after a week the obsidian is really well rounded off while the other stuff might need to run several weeks longer, so the obsidian gets set aside, maybe some more rough thrown in to take its place but I just build up the pulled out obsidian until I have enough to move it on in a batch by itself.
So maybe three of four weeks of doing this will yield a suitable load which then runs through the subsequent stages together, but this way they don't get damaged by harder rocks bashing into them later on.

I'll also use less grit for softer stones, where a 3 lb stage 2 agate load might get 3 full tablespoons of 120-220, I'd probably cut back to 1.5-2 tblsp for a similar load of obsidian, and add an extra couple spoons full of pellets.

Others will point out that you can often get away with mixing different hardness materials, but I think there are limits to that and I would generally try to avoid having materials that are much more than 2 mohs points apart rolling in the same barrel without really keeping tabs on them.