Physical wealth can be not only hard to accumulate, but also can
become hard to handle. Riches in small currencies, gold or lower, are
very difficult to handle. Large chests of gold are very cumbersome to
move. Fortunately there are metals which are more valuable then gold, 
and there are people who are more then happy to take their cut to
exchange it. It's not completely understood where they take the coins
and exchange them, but they get it done, and their results in platinum,
mithril and the like, are accepted at face value throughout the realms.
The following are the best types of coins you can come up with at any, 
depending on your skill%
  5% - zinc
 10% - tin
 20% - copper
 25% - bronze
 40% - silver
 50% - gold
 70% - platinum
 80% - anipium
 90% - batium
 95% - mithril