Mox Diamond is a mythic Artifact card from From the Vault: Relics (V10), collector number 10. Current market price: — (foil $1,730.26), TCGplayer market prices via Scryfall.
If this artifact would enter, you may discard a land card instead. If you do, put this artifact onto the battlefield. If you don't, put it into its owner's graveyard. {T}: Add one mana of any color.