I recently read a really good PV article on how to use planeswalkers. A friend of mine commented that a similar article regarding the use of Moxen would be a great help. While I fear I may not have the literary grace of PV nor his magical prowess I have played a whole lot of Moxen in my time. I was playing the heaviest Magic of my life around the era of Mirrodin and did not play a deck in any tournament that did not support at least 3 Chrome Mox. Obviously this article will focus on the cube usage of the various Moxen however much of what I say here will apply generally when playing and building with Mox.
There are four distinct types of Mox presently in Magic, the original five making one group, Jet, Sapphire, Emerald, Ruby and Pearl and then there are the unique Chrome, Diamond and Opal which collectively make up the fair Moxen but work differently as so must be looked at separately. I no longer play with the original Moxen in my cube, they are fun and allow a lot of cool things to happen and compared to the other power cards I have stopped using they are the most fair and ruin the least games. Despite this they are so outrageously powerful that every single deck possible in Magic would play at the very least the on colour ones. There is little to say about the original Moxen and you can't go too far wrong with them even if used in a noob like manner. Pick on colour ones incredibly highly with only other power or the absolute bomb cards like Jitte and Jace giving you pause for thought in terms of what to take. Only the most extreme agro decks and mono colour decks might not want off colour Mox in their list and only then when they have the nuts version. A good RDW isn't going to get much out of a non Ruby Mox as at least 80% of the mana requirement on all spells and abilities in the deck will be R not colourless. You don't need much of your mana expenditure to be colourless, say 35%, before off colour original Moxes become high picks, things that should be going 1st to 3rd in packs.
The original Moxes you do get you should play unless the afore mentioned RDW or similar situation occurs. To play these you should generally remove land however it is not always this cut and dry. Lets say you are a three colour Naya Zoo deck and you were going to run 17 lands and 23 spells but then you realised you picked up a handy Mox Emerald, Pearl and Ruby. By removing 3 lands and playing all three Mox you have increased your early mana potential however you will be regularly missing early lands drops and thus won't have much of a notable mana advantage on the no Mox version in the mid and late game. You will have hopefully gotten good mileage with the early mana advantage and potentially not need to have much mana later but this is not always the case, especailly if your deck is running any sort of powerful top end (four and five drops for the agro decks). In this situation you are best of cutting the least powerful card from your deck, perhaps even the two least powerful cards, usually one drops and keeping a reasonably high land count. Rather than just exploding out with three drops and staying at three mana for several turns you will curve out more normally but skip the first turn or two and thus have a huge tempo advantage right the way through to your finishers as opposed to just to the midgame. In this hypothetical situution I would play all three of my Moxen and either 15 or 16 land and then 21 or 22 other spells respectively (this final distinction would be down to the quality of my colour fixing and the power of my top end. More powerful top end and/or worse fixing lend to higher land counts. I think this is the only really pertinent advice I can offer on these simple, supremely powerful little cards. You can see them as a land you can lay without restriction and just cut lands which is fine to do and often correct in the really cheap decks like white weenie or RDW but in the more control or midrange deck you are often wiser to view the Mox as a Time Walk and replace a cheap spell with it instead. A Force Spike is something I chose to play as it gives me an OK card than has a lot of power on turn one when I am feeling vulnerable. This is the right sort of card to be replacing with a Mox if I feel that level of early boost is sufficient as the Mox will provide way more both on turn one and as the game goes on than the Spike will.
There are some great synergies with Mox such as Kor Shyfisher and there are some nombos such as Pernicious Deed, in the latter case you play both anyway as they are just so very good. Trinket Mage loves a Mox and Gorilla Shaman are great at breaking them. Decks with Upheaval love a Mox, or all the Mox! Decks with Armageddon, affinity, metalcraft, storm etc etc love the Mox. They banned artifact lands for being too good and the Mox have all the artifact lands perks as an incidental side effect and would still be first picks and in every deck regardless of that fact!
Mox Opal is the most niche of the Moxen as well as the one I have played with least, and exclusively in the singleton cube format thus making this part of the essay least relevant for non cuber readers. Rather useless I realize as it is the only Modern legal Mox but never mind, on with the show. Despite being the most niche it is also the most powerful Mox in a singleton format as it has no card disadvantage and taps for any colour. This is of course assuming you have metalcraft. When you can reliably and, importantly, at the stage of the game you most want the mana, be able to activate metalcraft then you should ALWAYS play Mox Opal. As I said, you always play an on colour Original Mox and this is better than that... Lots of decks have a high artifact count, or certainly sufficient to make the Mox Opal good but then you realize that most of those artifacts are near the top of the curve with little use for excess mana beyond that point and so the Opal is actually fairly useless. For all the artifact based combo decks and affinity/robots style agro decks the card is the absolute nut high. In most other kinds of deck it can be rather awkward, even in some of the less optimal artifact based ramp decks it can just wind up being less use than a land when you needed it in the all important early game. This is of course assuming you took the greedy route and played it as a good land and not a Time Walk by replacing a spell rather than a land with it.
The most interesting aspect of Mox Opal is that it is so powerful yet so dependant on your (cheap) artifact count that it has a very smooth curve of power in a variety of archetypes and deck lists. In some it will be blank and in others it will be one of the very very best cards possible in MtG with every possible power level inbetween accounted for. This means in a lot of decks the Mox will be roughly as powerful as the weaker cards in your deck list and will present some very interesting deck building options. In such situations you are essentially looking at taking a risk on the potential for the lucky high power outcomes rather than the blank card possibilities. You can consider ramming in a few more artifacts to increase the power of the Mox but this might well just be counter productive as the power level of the deck overall goes down due to the other changes. Ultimately I find it comes down to two things, one, do I need to be lucky to win verses what my opponents have? If I feel my matchups are good I will be inclined not to run the hopeful Mox, otherwise I will take the gambit. Two, do I have other non Mox uses I can put the card to? Do I have things like Tangle Wire and Smoke Stack that I just want to feed blank permanents too? Do I have a sac outlet I could use it for such as Goblin Welder? Do I have card quality and filter that I can throw it away with should it be miss firing? Finally, do I have powerful card draw mechanisms which can negate its deadness some of the time or abuse the extra mana with when I do explode? The more of these questions I can answer with a strong yes to the more inclined I will be towards running a hopeful Mox Opal. I can't recall what the lowest artifact count I have run a Mox Opal in a deck with but it was definitely below ten, which when a couple of them cost four of more, is more than just a hopeful Mox. I can't say either what rigid levels of artifacts you must have for it to become good / playable / bomb like. This depends on the synergies and support in your deck as well as the cost of your cards, the other artifacts in particular. Ball park I would say fifteen other artifacts with at least two thrids of them costing two or less in 40 cards and you are looking at a good card. You don't need to pick it very high though as so few other decks could dream of supporting it, eapecially given most of the appropriate support cards should be in your pool.
Chrome Mox is my favourite of the Moxen mostly because of the options it gives you compared to the others. With all the other Moxen you are either getting on predetermined colour or any colour you like, with Chrome you control the colour output. With Chrome and Diamond you have to lose a card in your hand to activate them however the choice is very easy with Diamond as to what to lose and has no bearing on the utility of the card once in play. Lands are lands and the worst one is usually obvious, the only real choice with Diamond is whether it is worth tossing the land and playing the Mox or keeping it and not. Chrome is easier to fit into a deck than Diamond but neither are easy cards to include sensibly within a deck list. Chrome is much like any other Mox and can simply replace a land card if you are inclined while this is not so much the case for Diamond as it needs lands itself and so commands a slightly higher land count.
So what is it about a deck that makes a Chrome or Diamond worth playing? Broadly speaking I would say three kinds of different thing potentially qualify your deck for one of these two card disadvantage Mox. One, the most common reason you can include one of these Mox is that your deck is full of card draw in some way, either an engine or just lots of powerful raw draw. Usually these decks are ones supporting Necropotence, Skullclamp and Land Tax or the symmetrical draw seven cards. Two, your deck is fairly non-interactive which is usually a combo deck but can be an all in agro or heavy disruption kind of deck. In these situations you have far more complete knowledge about what you can afford to lose in order to do your thing and so the frequency with which your Mox is viable to play increases a lot, this is especially the case with Chrome. These all in or non interactive decks do have a strong tendency to rely on the speed at which you make things happen as well which further increases the value of the Mox. Finally, three, decks that stand to gain a huge amount by being able to do a particular thing a turn early or having an extra mana at a particular point in the game. This may sound a bit silly as all decks are going to be keen to have an extra mana, in this case I mean much more specifically. A control deck reliant on having counter mana up from turn one due to lack of ability to deal with certain things. A mana denial deck where you need to be able to get those expensive mana denial spells down before they can cast serious things you are not set to deal with. The only time in history I can recall of red deck wins playing Chrome Mox is when Slith Firewalker was one of it's best threats and the potential to get it going on turn one was sufficiently powerful that it merited the card disadvantage of the Mox.
None of these are simple yes no answers and all come with a variety of shades, you simply have to judge when your deck has enough of these qualities that a Chrome or Diamond become viable. It is much like and increasing artifact count with Mox Opal but harder to judge. There are all sorts of sweeteners that have some synergy with your Mox which also encourage their play. Cards like Cataclysm, Upheaval and Armageddon all love a Mox go a way to making your deck more suitable for them should you be a little light on the more general reasons to play the Mox. There are other more subtle cards that like a Mox such as Kor Skyfisher, storm cards, Smokestack and Tanglewire, Balance, hellbent cards although most specifically Blood Scrivener, any card that involves a discard of your hand such as Liliana of the Veil or Wheel of Fortune, things that miss artifacts such as Death Cloud and so on that also help to make your Moxen get value. The two most common decks I find myself playing Chrome and/or Diamond in these days are black based Skullclamp and Necropotence decks and a wide array of mono blue decks. Both of these colours are fairly slow compared to what tempo Naya colours can lay on. To offset these both black and blue have far more raw card drawing potential. Blue in particular has very few strong cheap cards, notably for more permanent based strategies and by using Mox you can offset this deficiency and hope to skip that part of the curve.
So the next big question is when do you reach for Chrome and when do you reach for Diamond? Typically you want them for the same reasons and so in principle either would get the job done however usually one will be far more suitable than the other based on the nature of your deck. There are bunch of trends which will help you chose between the two. Chrome is better with fewer colours in your deck and can work well with low land counts as well as high. Diamond gets better as your deck includes more colours and not just for the obvious fixing but also because of the ratio of what you can pitch to help you out favours it as you increase colour count compared to Chrome. Diamond obviously operates better with higher land counts but awkwardly with lower mana curves. Should you want to boost up to two or three mana and stay there Diamond is likely better than Chrome while in the deck where you just want to skip one drops and carry on curving from two upwards then Chrome has the edge. When your deck is more about redundancy then Diamond has an edge while decks more about solutions favours a Chrome. Diamond discards a land rather than exiling it and so offers a bit of utility which can act as a sweetener for it, Deathrite Shaman, Tarmogoyf, Life from the Loam, Yawgmoth's Will and even Eternal Witness all have better synergy with Diamond than Chrome. Life from the Loam is also in another category of cards that make Diamond vastly more playable than Chrome, things like Land Tax, bounce lands, Treasure Hunt, Tithe and basically anything that offers card advantage but specifically in the form of land cards. I will basically never play a Chrome Mox in a white weenie deck as you have fairly little card advantage and the deck is a lot about redundancy. I will however fairly often play Mox Diamond as I have a lot of ways to get the lands I need and more plus the fact that my curve typically ends at four (and this is a key one, it means any land beyond my 4th is pretty irrelevant and so I can afford to lose one for an early boost).
There are those glorious decks than can run both Chrome and Diamond. I have done it in affinity which is a bit of a stretch in terms of using the cards as intended but given that cube affinity has so much artifact based synergy and the ability to pack a lot of card draw it is perfectly doable. This is of course on top of Mox Opal! Mono black Necropotence decks are the other place I find I can usually ram both into. It is an unusual deck in that it operates well off both low mana and few cards yet has the capacity to explode and run well off lots of cards and lots of mana. This makes both Chrome and Diamond good at giving you a tempo and options boost in the early game without overly gimping you on the things you will need later on while allowing you to go off later in the game in a more explosive way at minimal or no cost. Turn one Sinkholes, Hymns to Tourach, Pack Rats and Dark Confidants also add appeal to both Mox in question for the Necro deck.
The power and versatility of the original five Mox demonstrate very well how powerful Opal, Diamond and Chrome can be when you effectively negate their downside. Knowing when you have a deck that is able to negate the drawbacks and that has the sufficient qualities to be able to include one of the fair Mox is the hard part. For those areas my article failed to shed sufficient light on which Mox and when my advice would be to air on the side of playing the Mox you think is right. You can only learn from experience and by getting it wrong, if you never take the plunge and risk that Mox you will never master the art of Moxing and never be able to fully unleash their power on your hapless opponents. Even in the best decks for them you will still have them be the worst possible top deck. I notice the fair Moxen going very late in MODO cube drafts, I think this is in part due to inexperience with how and where to play them well but also about human bias and association. With Chrome and Diamond you always feel a little hurt when you play them because you have to lose something to do so, once you have them down you basically forget about them as they are a boring old land. The times they stand out most are when you draw them dead in the late game and they are more of an insult than a basic land at that point. Our memories of the Mox tend to be negative ones even when it has done great work for us and won us several games. We often accredit the win to the threat and not the mana that got it there for us however we are more than happy to blame the Mox for our losses. Certainly the Mox are not for every deck but I feel they are an undervalued and underplayed card by the Magic community. They are among the more niche cards in the cube yet they still utterly deserve their place in any good one!