Golgari Cluestone

The two-color mana rock that knows its own expiration date. The Cluestone cycle solved an old fixing problem: a rock that ramps and fixes early is dead weight in the late game, when you have lands to spare and want gas instead. The sacrifice clause folds the rock back into a card draw once its ramping job is done, so the slot never goes truly dead. The cost structure is the tell on how much that flexibility is worth: cashing it in demands both of its colors plus the tap and the sacrifice, which keeps the cantrip from being a free afterthought and forces you to actually have the mana you were supposed to be fixing in the first place. It sits in a long line of guild-aligned ramp pieces that try to stay relevant from turn three onward, a slower cousin to the more aggressive Signets and the slightly cheaper Keyrunes that swing into bodies instead. Among that family, the Cluestone is the one built for grind: less explosive, more insurance against flooding. Plain in execution, but the design idea (a fixing rock with a built-in second life) is a clean answer to the question of what a mana rock should do once the game has moved past needing mana.