You can get very hard gold alloys, also in higher karats.
To my best knowledge dental gold alloys are gold and platinum (90/10)
or gold and palladium (same proportion), and these are - for the
purpose - very hard indeed.
Another thing that makes gold very hard is to alloy it with tin (Sn -
Stannum, the major ingrediens in pewter). Use it very clean,
analytical grade and replace some of the silver in your alloy with it
to get the approximately same colour. Tin is considerably cheaper as
Pt or Pa, even in the analytical grade.
I think you will have to try alloying a wee bit till you get a result
that fits your needs.
Niels L�vschal, Jyllinge, Denmark
phone (+45) 46 78 89 94