Promising Vein is a common Land — Cave card from The Lost Caverns of Ixalan (LCI), collector number 279. Current market price: $0.28 (foil $0.31), TCGplayer market prices via Scryfall.
{T}: Add {C}. {1}, {T}, Sacrifice this land: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.