Ok... I have a spring loaded drag engraver I use occasionally for misc
jobs. I originally used diamond tipped stylus in it, but the diamond tip
would snap off on some pretty insignificant irregularities. When I needed
to do some stuff once, and did not have a good diamond tip on hand I chucked
a broken carbide end mill in the lathe and used a fiber wheel to
grind/polish it to a nice point. It worked well enough for the application
on brass. I have since used it on aluminum, brass, and steel (tests on the
side of a socket). With some steels it take 3 or 4 passes to make a nice
image, but it works. What do the diamond tip drag styluses do that this one
doesn't?

Well, on some materials (like brass) the carbide one does leave a rough feel
if you run your hand over it.