Is it a pen or a pencil?
I'm lab pen, pencil, sharpie, exacto crazy.
In my lab coat I always have a retractable exacto, sharpie, fine point sharpie, and my all stainless steel .05 red led mechanical pencil. It's metal so it never melts when I leave it next to the waxer. Sharpie is great on die stone, but stone separator will always make the ink bleed a mess. But I don't think you'll be using separator in ortho lab.
So if you want to write pan numbers and patient names neatly on stone, get a fine point sharpie.
Or you can use the mechanical pencil, and then seal the writing in with die hardner.
I used to write pan numbers real small and neat on all my dies and seal it in with die hardner.
Oh and I have an awesome 7 year pen always on me. It's spose to write a meter a day for 7 years. It was a gift, but damn that thing holds up...