[Jack]

There are some tumblers and supplies on the market that are made for rock polishing rather than case tumbling. The media you put in a tumbler for rock polishing will have abrasives that can cause wear on cases-not good.
Any tumbler made by a reloading company like RCBS, Lyman, Midway, Dillon, etc is designed for case polishing, and so is the media they sell for them- perfectly safe to use. Tumbler media used for case cleaning is generally either corncob or walnut shells.
That said, tumbling cases isn't mandatory. It does keep your cases clean so that they don't drag crud into your dies, firearm chamber, etc, and makes any case defect easier to spot.
I've seen the Iosso liquid stuff advertised but I've never used it.