Seeking dark metal

Hello,

I am looking for a metal that I can cast that is naturally a dark
grey so when it is polished it has a gunmetal finish. So far I have
researched black gold but I assume it would be very expensive, is
there any other metal out their that I could use? I want to stay away
from patinas since this will be cast as a ring and patinas tend to
wear away.

Bradley B.