Silver
Silver, a rare metal that is valued less than gold--12 silver per gold--is generally beneath the notice of nobles (who receive gold coins for their tax bonds) - although certainly a noble might have a lot of silver and silver objects.
Silver is the primary currency of adventurers, who find it by killing Monsters and Undead, as well as by selling various Items.
Commoners and peasants also have silver, though their primary currency (what they pay to their local lord) is usually Food. They also probably use the copper coins which are beneath the notice of even adventurers.