Question: I have been world-building a setting where a species of advanced aliens invades Earth, only to be forced off after the humans resort to guerilla tactics. In this setting, biological life is abundant in the universe, so Earth is not unique in that regard. The aliens are advanced, but not to the point where they have outgrown the need for raw materials. In the story I have now, the reason that the aliens have not wiped out Earth yet is because using nuclear/chemical/biological weapons of mass destruction might make the earth uninhabitable for a future harvesting operation. I have not determined what resource would best fit this role. However, I have discerned that it has the following properties:

* It is inorganic
* It actually exists in real life
* It is in reasonable abundance on earth, but is rare throughout the
universe has a whole
* The material cannot be reasonably synthesized under all known laws of
physics
* It is valuable enough for an advanced civilization to fight a war
over it, but not so valuable that the civilization cannot survive without it.

Which material, if any, would best fit the above criteria?

Answer: 
> 
> Look at the stunning color variations, the beautiful discontinuity of the impurities, all the gorgeous minute imperfections! *This*, my dear gentlebeings, is a pristine certified-natural carbon crystal, retrieved with blood, eyes and tears from an uncivilized pre-synthesis (of course!) waterworld by the great M'gr!hgk^6 themselves!
> 
> 
> None of this synthed nonsense you see all over, no no, this is the real deal! Comes complete with a provenance guarantee, a Millennium-class vacuum showcase, *and* a detailed 6D recording of its recovery!
> 
> 
> Minimum bid set to six thousand credits! Do we have a first bid? Ah, six thousand one hundred credits from the seven-tentacled gentlebeing in the middle pond. Do we have six thousand two hundred? Right, ...
> 
> 
> 

Your aliens are not scavengers or miners, they are high-class *collectors*. The more *pristine* and *untouched* the item, and the more story-worthy its provenance, the more valuable it is.

So, no nuclear weapons ("*Do you have* any *idea what neutrons can* do *to an item?*"), no planetary-scale strikes, and that collection flotilla had better not have a single synther anywhere, lest doubts be cast on the provenance of the items.

And, sure, you can just mine an asteroid somewhere, but that's just *boring*. It's not as if the 'roid would go anywhere. Nothing like saving a precious item, *just before* the barbarians on its homeworld destroy it.
Humans have attained incredible levels of technology without unlocking the secret to FTL travel. By a similar coincidence, these aliens attained incredible levels of technology without unlocking the secret to integrated circuits and powerful computers.
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In the first stages of the invasion the aliens hit major cities with orbital bombardment, not knowing that computers even existed. As a result, the fabs and most of the humans who know how to create them are dead - Gigahertz class computer chips became a finite resource overnight. However, in the ensuing conflict it became very clear to the aliens how useful computers are in war.

After they won, they incorporated human chips into their weapons, and are now scouring earth for computers, game systems, and phones to repurpose into the new intelligent weapons that they are exporting to the front lines of other conflicts they are fighting in the galaxy. Of course, given another 70 years, they could probably duplicate the tech to make chips by torturing captured scientists and reading our books. However, they don't need chips in 70 years: they need them now to win the war with the Zorblaxians