Mox Diamond is a rare Artifact card from Stronghold (STH), collector number 138. Current market price: $1,117.50, 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.