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: 
### No FTL? No invasion. No physical trade.

The economics of sending even a trading ship would mean no mission would be sent. Time frames are on the order of tens of thousands of years(or more) and ships would cost trillions of dollars make this an economic non starter. The people who launch the ships would never see the benefits. There descendants might.

Additionally how could they possibly know that a 'natural' rare resource exists? The rational thing would be to assume a mostly uniform universe.

### What resource could aliens want? Earth Culture.

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

Earth culture. Unique to earth. Not critical for survival of aliens, it exists. It is non organic. Earthlings fight wars over earth culture.

Is this worth Aliens sending non FTL ships to acquire? Not when they can ask/trade for large sections of it via photon based communications. Less risk, drastically higher return on investment, more ethical(from human perspective at least).

### FTL changes everything.

IF FTL exists this changes everything. Then the music, art other intellectual property would be lucrative. Plants that were not toxic might be useful trade goods to provide new food items.
Humans
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Just like humans collect birds, fish, snakes, and other animals, and are willing to pay large amounts of money for rare specimen, aliens collect species from other planets. The further away it comes from, the more space bucks they're willing to pay! A space ship full of humans will earn the crew a lot of money.