Gemstone Caverns

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.
{T}: Add {C}. If Gemstone Caverns has a luck counter on it, instead add one mana of any color.

Edge of Eternities: Stellar Sights

Tales of Middle-earth Commander

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 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.