Hall of Gemstone
A symmetrical lock dressed as a green enchantment, and one of the cleanest examples of the World supertype doing real strategic work. Because only one World enchantment ever occupies the battlefield, this is a piece you build around rather than a card you stumble into: the player who casts it sets the terms on their own upkeep, while everyone else has to navigate the same constraint on theirs. The mechanic punishes multicolor manabases by collapsing lands' colored output to a single chosen color per turn, which makes it a stax piece against greedy decks while a mono-color shell shrugs it off. The timing detail is the whole design: the color is chosen anew each upkeep, by the active player, so the lock is not a static prison but a rolling one that each player resets to their own advantage. That self-correcting symmetry is what stops the card from becoming a one-sided cage; all of the asymmetry lives in the deckbuilding, not the text. Green getting a tool that taxes color-fixing rather than ramping into it was an unusual assignment for the era, and the card has aged into a quiet staple of color-screw and stax-adjacent strategies precisely because the constraint it imposes scales with how greedy the table is.
