Supposedly, polishing makes em less valuable but it's not like I care much.
Are there coins available that are made with metals different than silver or gold?
You mean that I collect? Then no. But generally yeah, coins have been made from all kinds of metal, even aluminum
Have you purchased any gold coins?
I was lucky enough to inherit a significant amount of gold in various forms (mainly jewels), atm not much into it, maybe in the future :)