In another adventure, the party found tons of copper in oxide ingots. It worked out for the party as said party was looting the tomb of a city founder - on the city's behalf and the city minted all that copper into fat copper coins...
(the recap, if anyone cares:
"4: Grave Hunt: The scribe Ophelos has found an old parchment indicating the true location of the tomb of Selecus I Nicator, founder of Zeugma (one of Alexander's generals). The city approaches the party to go on an expedition in exchange for a share of the treasure. After a few days of travel, the tomb is found inside a barrow which the locals swear is haunted. After defeating the goblins inside (who were tricking the peasants into making offerings) and the undead guardians, the tomb is secured, will with many historical artefacts and quite a lot of treasure, including the griffin's claw, an enchanted sword made of Damascus steel."
... looks like at the time I didn't consider the detail about the copper worth writing down!)