If multiple Gemstone Caverns are put onto the battlefield under a single player's control before the game begins, the "legend rule" won't put the extras into that player's graveyard until just before the starting player gets priority during the game's first upkeep step. There's no opportunity to tap the extras for mana.
2021-03-19
A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.
If this card is in your opening hand and you're not the starting player, you may begin the game with Gemstone Caverns on the battlefield with a luck counter on it. If you do, exile a card from your hand.
: Add . If Gemstone Caverns has a luck counter on it, instead add one mana of any color.
The free acceleration land. When it works, Gemstone Caverns puts a land on the battlefield before your first turn for free — effectively a Mox that fixes any color. In a four-player pod, you're not the starting player roughly 75% of the time, so it triggers often. That ramp turn-zero is huge for fast combo and high-power decks racing to deploy a commander or assemble a win.
Who wants it: cEDH and competitive decks where a one-card-discount on speed matters, especially fast combo, Storm, and any deck that wants to land its commander a turn early. The any-color fix is gravy for greedy multicolor manabases.
The catch: it's only great in your opening hand off the play-loss — otherwise it's a colorless tapland that enters untapped but produces nothing useful and can't be improved. You also pay a card to "free-cast" it.
Skip it in: casual/midrange decks that don't capitalize on the early tempo, and budget builds where the price tag isn't worth a marginal colorless source.
: Add 
