Dimir Locket

The Locket is the second-generation guild mana rock, and its late-game conversion clause is the whole justification for running it over the cheaper Dimir Signet. A three-mana rock that taps for one of two colors is unremarkable on rate; the design bet is the back half, which lets the rock cash itself in for a pair of cards once the manabase has done its job. That sacrifice is deliberately expensive in pips (four hybrid symbols), so the conversion is a release valve rather than an early play: you run the Locket as fixing while the game is being built, then redeem it for cards once your draws of mana matter less than your draws of action. It is a clean answer to the perennial problem of the mana rock that becomes a dead topdeck, recycling itself the moment its first job is finished. The hybrid cost on the draw ability is the quiet flexibility that makes it work across decks leaning either color: a heavily blue deck and a heavily black one can both pay it without warping their land base. As designs go it is modest, but it is the kind of modest that survives reprinting: a fixing-plus-insurance package that asks nothing of the rest of your cards and rewards patience rather than construction.