At a portal or trade portal, a single gold coin can be exchanged for 100 copper coins. The reverse is also possible, exchanging 100 copper coins for one gold coin. This means that by turning them into coins and back, copper ingots can be turned into gold ingots (at a rate of 100:1).
Starting in version 1.6, gold coins can also be exchanged into platinum coins at a 100:1 ratio. This means that by turning them into coins and back, platinum ingots can be turned into gold ingots (at a rate of 1:10).
Prior to version 1.4, one gold coin could be warped in from a portal in exchange for one time crystal.
As of version 1.6, gold coins can be exchanged with platinum coins at a ratio of 100:1 (gold:platinum). Gold coins also received a new slogan in version 1.6, replacing the old slogan: "For trading."