Not legal in Standard, Pioneer, Pauper, Alchemy, Standard Brawl, Pauper Commander, Premodern, Old School, Penny Dreadful
Notes & Rules Information
If you control Mox Opal and one other artifact, then have a second Mox Opal enter the battlefield as your third artifact, you can't activate either one's mana ability before putting one into your graveyard due to the “legend rule.”
: Add one mana of any color. Activate only if you control three or more artifacts.