RESTRICT: The card is a non-land card that costs 1 or less, taps to add mana to your pool the turn it enters the battlefield, and is castable on turn 1.
Rationale - Cards that accelerate mana too quickly are broken and can end games, virtually or actually, far too quickly. The mana disparity gained in one turn would likely lead to games of solitaire or require all decks to run 8+ pitch counters just to survive to their turn 1. Restricting such pieces create enough variance to where broken openings will happen, but can't be counted on consistently enough to lead to a coin-flip format. (This allows lotus bloom and tantalite to live, but hits chrome mox and opal as well as the ABU moxen, black lotus, mana vault, and sol ring.)