Well, I don't polish rocks or anything, but I think the type of tumbler used is the rotary type, not the vibrating type. The rotary type works just like a smaller slower version of a laundry dryer, as I've seen. Now, if you put some media in a rotary tumbler, and a few spoons of dry chromium oxide, I don't think it would have any problem resulting in an even coating after a few tumbles.

Heck, the dry chromium oxide doesn't have any problem giving my kitchen counters an even coating all by itself if I'm not careful when applying it! I don't see the need for it being wet, and if it did have anything else in it, it wouldn't be pure chromium oxide, would it? It'd be chromium oxide AND whatever else.

I have both the hand american dry chromium oxide and the liquid based, and I've never had anything that wouldn't take the dry just fine.