My personal favorites are one I made off of a piece of vinyl and a rock from my yard. The vinyl stamp looks good for skin and to just break up an otherwise flat area. The backside of it ended up with a rough surface that was because my latex was going bad. That ened up being good for texture too.
The rock can smash things... or also make a boken up surface.
I’ve used it on my medium sculpting clay also, that pic is Klean Klay regular. What I end up doing is heating the area up a little with a heat gun. I’ve got to be careful not to actually melt it but that depends on the area’s detail. Once it cools a little then it will be more evenly warmed and softer then it's ready to accept a stamp. I don’t use water based clay but I’d imagine hardness wouldn’t be an issue.