Mox Diamond

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.

Tempest Remastered

If Mox Diamond would enter the battlefield, you may discard a land card instead. If you do, put Mox Diamond onto the battlefield. If you don't, put it into its owner's graveyard.
{T}: Add one mana of any color to your mana pool.

From the Vault: Relics

Stronghold

When Mox Diamond comes into play, choose and discard a land card or sacrifice Mox Diamond.
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T: Add one mana of any color to your mana pool. Play this ability as a mana source.

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.