Hall of Gemstone is a rare World Enchantment card from World Championship Decks 1998 (WC98), collector number bs221sb. Current market price: $5.34, TCGplayer market prices via Scryfall.
At the beginning of each player's upkeep, that player chooses a color. Until end of turn, lands tapped for mana produce mana of the chosen color instead of any other color.