Mox Emerald

A free artifact that produces colored mana breaks the curve, and the original five moxen are the foundational proof of the design problem that has shaped artifact mana ever since. Every land slot this replaces is a turn of tempo banked, every opening hand it appears in is functionally one card larger, and the green it produces sidesteps the entire reason fixing is supposed to cost something. The design has been circled for decades without an unrestricted reprint: Chrome Mox asks for an imprint, Mox Opal asks for metalcraft, Mox Amber asks for a legendary permanent, Mox Diamond asks for a land discard. Each of those restrictions is a tax invented specifically because the version with no clause attached (this card) warped every format it touched. The Reserved List froze it in place as an artifact of 1993's design intuitions, when the team did not yet know that "free" and "colored" could not coexist on the same permanent without consequence. Green's slot in the cycle landed on the color that historically does its own ramp through lands and creatures, which makes Mox Emerald the most redundant of the five in pure function and the most revealing about what the cycle was actually selling: not green mana, but the first turn it arrived.