Crystal was removed because it was uninteresting at best and annoying at worst.
This is a good think overall, but I believe there is something interesting that can be achieved with multiple resource's types.
Suggestion 1* Each ship costs either only metal or only crystal. 1 crystal is worth 1 metal.
* Each planet has either only metal deposits or only crystal asteroids.
* Half of the ship types requires metal, the other half requires crystal.
* Each planet has exactly 1/2 chance to have metal and 1/2 to have crystal.
* One is not rarer or more desirable than the other.
* According to the ship types a player has, a planet's resource type can make it more or less desirable.
* According to the resources a player has, a capturable ship type can be more or less desirable.
* Converters can be built: each convert N resources to 1 other resource at a given rate and consume some energy. (N=3?)
* Homeworlds are special: they are half metal and half crystal (and only have an even number of resource spot). AI homeworlds follow this rule.
* Captive/Home human settlements have a corresponding Captive/Home human mining colony with same AIP cost and production rate that produce crystal instead of metal.
* Orbital scrap on human planet is properly tracked: a crystal ship adds its scrap to the crystal count.
* OCStations gather crystal and metal scrap independently.
* AI doesn't care about scrap being metal or crystal. It's supposed it can freely convert one into the other at a 1:1 ratio.
* This feature can be disabled in the lobby (in the "game option"): all ships cost metal and all planets have only metal deposits; converters cannot be built; all Captive/Home human mining colony are replaced with the corresponding human settlement.
* The ship's definitions (xml?) need to specify the type of the price (metal or crystal). All crystal prices are automatically converted into metal at load time if the option is disabled.
* How to assign metal or crystal to a ship remains to be defined.
* The split may be made over a tactical difference, like between small, fast ships and large, powerful ships (most starships would be of the same resource type, then).
* The split may be totally arbitrary, but then preferring one resource over the other is no more a meaningful strategy.
* The split may be made over hull type, but that needs to be done carefully to split the ship types in two equal parts.
Suggestion 2: further!* If this game option is activated, a third resource type is added to the game.
* Each planet now has 3/7 chances to be metal, 3/7 chances to be crystal and 1/7 chance to be that third resource.
* 1/7 of ship types in the definition files have a cost type set to this third resource; if it's deactivated, they cost metal or crystal (they have a fallback tag); if crystal is deactivated too, they cost metal.
* Same proportions for planetary resource type. Same scrap/reprisal rules.
* However, there is no converter for the third resource, neither in nor out. Only metal-crystal conversions are allowed.
* Then, players rarely need this third resource, but if they choose to use (capture, hack, etc) they need to capture (and keep) a planet with this resource.
* This special, rare resource could be assigned to experimental ships or some with really special abilities (in the vein of the Viral Shredders, maybe the Leech starship, etc).
* On the name, it could be "advanced crystal", "liquid zerkanium", "titanyradium", ... I don't know yet.
* On the color, let's list them: metal is brown/orange, energy is yellow, science is blue (cyan), hacking is green, AIP is gray.
* Crystal used to be green, which now conflicts with hacking.
* Available colors are purple, deep blue, red... and that's all, basically.
* As purple is usually used for high rarity items (
http://www.giantbomb.com/color-coded-loot/3015-4702/), purple seems more atuned with this rarer resource.
* For the smallest change, crystal could get deep blue.
* If slightly bigger change is accepted, crystal could get back the green color and hacking could take dark blue. This way, crystal would have its original color and the two "virtual" resources (science and hacking) would be two tints of blue.
* While I'm talking about color, I think AIP could be better in red because it's a negative "resource". It would be a dark red to avoid making it too bright in the HUD. Metal could also get a more reddish/orange/rusty tint to make it stands out more next to the pale yellow Energy.
Personal notes* I rather like the Spirecraft MF and it's mining mechanism, but I never play it for several reasons.
* First, it only gives powerful things. At least they are less straightforward than the golems and more subtle to use, but they still are superweapons that need exowaves to balance and they bring the game to a higher powerlevel. In the emergent gameplay of AIW, I feel superweapons mostly crush meaningful choices. But that's more a personal taste and not a truly important matter.
* What is more important (IMHO) is that asteroid types brings only differences in power, no difference in kind. There is no choice to do about capturing one or another planet in regard of its asteroid load: it's only a matter of quantity and quality. I think it would be much more interesting to have planets' resources bringing differences in kind for setting more interesting and meaningful choices.
* What if each planet has between one and three types of asteroids and each spirecraft requires a specific type?
* Well, a tons of balance tweaks are needed for setting up that variant of rule (like how to determine the mark of the spirecraft? how to deal with the ship cap? etc) but it would make planet capture choices much more interesting, I guess. (Maybe a mean to steal some asteroids without capturing the whole planet would be required...)
* Anyway, this is far off topic. Just an example of how I would like to do mods and why I like the idea of different resources, despite the added complexity. That added complexity was removed for being "just annoying" and I totally agree with that.
* If that crystal / third resource idea proves to be even more annoying while bringing no interesting and meaningful choices, then I would have failed to design a good mod and it would fall into oblivion.