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.
Item Description Edit
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.