Coal Mine/Diamond Mine

{1}; Put a coal counter on Coal Mine if it's tapped.
{t}; Flip Coal Mine into Diamond Mine if it has
no coal counters on it. Otherwise remove all coal counters from it, add that much mana to your mana pool, and put a diamond counter on it.
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At the beginning of your upkeep, if Diamond Mine has no diamond counters on it, flip it.
{t}, Remove a diamond counter from Diamond Mine; add a mana of any color to your mana pool.