If Gemstone Caverns is in your opening hand and you're not playing first, 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.
7/1/2013 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 first player gets priority during their first upkeep step. There’s no opportunity to tap the extras for mana.
6/8/2016 A player’s “opening hand” is the hand of cards the player has after all players have taken mulligans and “scryed” if applicable. 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.