I would never encourage someone to make their mark/purity of metal
upon jewellery while it was hot due to the likeliness of deforming
it... metal being more malleable when hot. Their are hand punches
available from Rio Grande... both bent for ring shanks and straight.
They carry common gold carats and a sterling silver one if memory
serves, also they will make one to provided specifications (for your
mark, etc, I presume). I suggest a light hand when punching the
metal, but with a few practice runs you'll become comfortable and
get the idea. I think that a pliers style may be available as are
mechanical engravers. I hope I didn't misunderstand the question.
Fredericton, NB, Canada