Copper Ore is one of the items used to upgrade your tools in Harvest Moon: Friends of Mineral Town.
Copper can only be mined in the Spring Mine. It is relatively invaluable, and can be found on all floors of the mine.
Inventory Description: "Copper: Found in mines. Used to upgrade Tools."
Sell price: 15G
Gray is known to be partial to ores of all sorts, including the copper ore.