Those are moldable foam pieces. Heat gently, press on any dimensional thing, and the foam molds to the shape. Ink and stamp. Reheat, and the blocks smooth out and can be used again.
I have even made a negative with one block, let it cool, then use a second block to press on the first to make a positive stamp. Fun stuff.
When I'm not near the stamp I love, I love the stamp I'm near.
My tiny little gallery