Gold Mine

At the beginning of each player's upkeep, that player may exile a creature he or she controls.
At the beginning of each player's end step, that player may flip a coin. If he or she wins the flip,
he or she returns a creature card he or she owns exiled with Gold Mine to the battlefield under
his or her control, then creates a colorless artifact token named Gold. It has "Sacrifice this artifact: Add one mana of any color to your mana pool."