Gemstone Mine is a special Land card from Time Spiral Timeshifted (TSB), collector number 119. Current market price: $5.37 (foil $167.99), TCGplayer market prices via Scryfall.
This land enters with three mining counters on it. {T}, Remove a mining counter from this land: Add one mana of any color. If there are no mining counters on this land, sacrifice it.