Marco, Most “standard” air grinders run at about 20,000 rpm NO LOAD.
Harbor Freight sells a pencil grinder that is supposed to do 50,000
or 80,000 rpm, for ??$80.00??. There are costly industrial air
grinders available for $$$$$$ that run 100,000+ rpm, that look like a
regular air grinder.
Another option is to get a flex shaft hand piece that is sold as a
"high speed" handle. That is what was recommended to me at a jewelry
demo/school/selling sort of show. Also at this show they were
recommending high speed, variable speed, electric motor in the hand
piece, expensive units. I ended getting the flex shaft high speed
handle, but have never used it.