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.