Izzet Signet

The signets answered a question older mana rocks fumbled: how do you ramp and fix in the same card without paying full price for either? Spend one generic, get two specific colors back. That is a net of plus-one mana, but the colorless mana you feed in is the cheapest thing to spare and the colored mana you get out is the resource a two-color deck actually needs on turn two. Mind Stone and Worn Powerstone before it added raw mana but no color control; the signet does both, which is why the cycle became a deckbuilding constant. It smooths a two-color manabase without an upkeep cost or a one-shot sacrifice, and the activation loops cleanly through untappers and cost-reducers when a build wants to push it past simple fixing. The Izzet pairing slots into the colors most inclined to hold up interaction and cast at instant speed, so the filtering can happen on an opponent's turn in response to whatever they do, converting a spare colorless into the blue or red a counterspell or burn answer demands right then. It is the least flashy card in any list that runs it and one of the first an Izzet pilot reaches for anyway, precisely because it never asks for a build-around: it just makes the second color reliable.