The concept of Mox is inherently linked to the very beginning of Magic: The Gathering and brings with it an air of myth that even the newest Magic players are able to recognize more than a quarter of a century later.
Artifacts for zero mana that can replicate the function of a land, while not being limited by the “one land drop per turn” rule are the supreme mana rocks and Moxen are the accelerants of choice in every format where they show up. Since the original quintet got permanently relegated to Vintage, the designers took it upon themselves to come up with new and intriguing ways to rekindle the Mox flavor without incurring the same kind of sanctions – inasmuch as the very prospect of a free mana boost is an intrinsically dangerous proposition. The eighth black-bordered attempt at this feat (following the notorious five precursors from Limited Edition Alpha (August 1993), then Mox Diamond from Stronghold (March 1998) and Chrome Mox from Mirrodin (October 2003)) diverted from the pattern set by the latter two, which had both tried to limit their impact by coming with built-in card disadvantage.
Mox Opal, instead, associates itself with its set’s signature artifact-based mechanic of metalcraft. This new approach will prove extremely successful and will be repeated - in diminishing returns - by the next Mox in line, the legend-based Mox Amber in Dominaria (April 2018).
Still by far the most expensive card from Scars of Mirrodin (October 2010) and actually the most expensive Mox on Magic Online (due to the original Moxen having been reprinted within the MTGO-only set Vintage Masters), Mox Opal reignited the old dream of dropping a free mana rock and immediately exploiting its boost, without having to give up any other resource in the process. More so, it encompassed all colors of mana, something even its illustrious ancestors did not dare. Furthermore, it did so in a way that didn’t result in a restriction in Vintage, since its Legendary supertype already kind of restricts itself to a single copy at a time. (However, shenanigans can be envisioned with a second copy sending the first to the graveyard after it is tapped for mana, only to recur the whole cycle in an infinite loop.)
All of this is accomplished through metalcraft, which is the very route that allowed the return to Mirrodin to bring back the feeling of a metal-clad, automaton-filled world, without revisiting the excesses that affinity produced the first time around. Still a powerful mechanic in its own right, metalcraft is sort of a redesigned affinity, in that the crux of it remains as having artifacts on the battlefield to generate further advantage. But the scope of such advantage changes from cost reduction to the achievement of a superior effect or the unlocking of an ability – in this case Mox Opal’s all-colored mana production.
Of course, it soon became clear, once again, that putting cheap artifacts in play is awfully easy, especially with cards like Ornithopter and Memnite also dropping for zero and Mox Opal counting itself among the triad of artifacts it needs to attain metalcraft. This naturally conduced to the same kind of Robots lists that were first launched by the original Mirrodin block.
But Mox Opal’s career would prove wider than that. It’s certainly a centerpiece in Affinity/Robots decks in Modern, but also crucial in all variants featuring Hardened Scales, as well as in decks with Urza, Lord High Artificer; in Legacy, Ad Nauseam decks run many cheap artifacts like Lion’s Eye Diamond, all of which make great companions for Mox Opal; and even Vintage itself is not above enlisting Mox Opal’s help to better prepare for a Paradoxical Outcome.
Inevitably, when a Mox is in the building, the game will take notice.