Card
Mox Diamond
Artifact · MV 0.0
Oracle text
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.
Rulings
2008-05-01 · If you don't discard a land card, Mox Diamond never enters. It won't trigger abilities that look for something entering, and you won't get the opportunity to tap it for mana.