Take it from someone who was both an avid shooter/reloader and rock polisher. I have both rolling and vibrating ones. Brass, metal polishing vibrators use very light dry medium, crushed walnut shells or corn cob, etc...by volume, about 3/4 medium to 1/4 brass, etc.
One of the problems with those tumblers and rocks, is the volume needed to get the tumbling action working properly. The bowl needs to be at least 1/2 full. Rocks by volume are far heavier than the same volume of brass casings mixed with dry medium.
Another problem is motor sizes and materials the bowls are made of. In general, motors on metal polishing units are not near powerful enough to get the required action on a bowl of rocks. Also the bowls are
generally made of thinner and more flimsy than rock tumbling bowls and wear out much faster.
The Lyman brass reloading tumbler may be the exception on sturdy bowl construction, but the motor is far too under powered to work with a bowl of rock, 1/2 full. The Harbor Freight bowl is massive but flimsy thin plastic. The motor would be powerful enough if you mounted a smaller rock tumbling bowl on it...but that's not cost effective.
Y'all may not agree with me, but that is what years of experience has taught me.