I get quite a few orders for stamps to use for clay projects.
People have used them on pottery, Sculpey, Play Doh, Precious Metal Clay (PMC) jewelry, and tiles.
I make these stamps extra deeply etched so they leave a good deep impression.
They can be used with a block for flat surfaces or pressed by hand without a block to work around curved surfaces.
Another neat idea is to stamp on clay using slip as the "ink" (see this link)
There are quite a few interesting videos on YouTube to give you some more ideas for stamping on clay.