Mox Diamond is making moves. Mox Opal is at a minimum that it won't see again for some time.
The second-tier Moxen (Mox Diamond, Mox Opal, Chrome Mox) should always be on your short-list for cards to watch.
We start with the most recent,
Mox Opal sees play in a somewhat limited archetype (artifacts), but a timeless one which spans every Eternal format. With metalcraft activated, Mox Opal is marginally more powerful than the original Moxen (any color). This just saw a reprinting in Modern Masters 2015. At $35, this is the lowest it will be for years to come, though the uptick will be gradual to start (keep an eye on this one). This is a low-risk, medium-yield, medium-long-term pickup.
Chrome Mox is arguably the worst of the three, with the significant drawback of a card exiled from your hand (though it is good, and still undervalued, in my opinion). Don't invest in these right now, but don't forget about them (new Eldrazi exile shenanigans might interact, for example). Graph omitted! Wham!
Mox Diamond has a "discard a land" drawback, but lands in graveyards are oh so delicious in Legacy, Vintage and Commander. This makes Mox Diamond significantly better than the original Moxen in some specific situations, around which one may design a deck. Though not Modern legal, of the "non-power" Moxen, Mox Diamond is arguably the most widely usable (everyone uses lands) and the most breakable (with cards like Crucible of Worlds, or other triggered effects). These are very-low-risk (these are on the Reserved list!), medium-yield, long-term pickup. Mox Diamond is demonstrably the oldest and scarcest of the three "modern" Moxen...
...and it's putting on some fresh pants.