Here is a web source of some procedures for metallographic etching.
Includes silver and other precious metals but not gold.
You might check :
G. Petzow, Metallographic Etching, ASM (American Society for
Metals), 1978. I don't have a copy. For other than pure gold you
can etch out the alloy metal and expose the gold grains, but I
don't have a cookbook procedure.