Sci-fi Treasure

Treasure is easy in fantasy realms. Usually, it's great piles of gold coins, gleaming gems, and works of art. If you're doing a more Bronze Age thing, it can be cauldrons and tripods and drinking vessels like you see in The Odyssey. But what's treasure in a sci-fi universe? Large stacks of credit-vouchers just don't have the same feel as tumbled piles of doubloons and ancient crowns.

Back in the middle of the 20th century, when the future was nuclear, space powers feuded over fissionables the way 20th century powers fought over oil. Later, when the power of the future shifted from fission to fusion, He3 became the thing to fight over.

There's been all sorts of unobtaniums associated with power generation or FTL travel, such as Star Trek's dilithium crystals or the floaty magic rocks in the Avatar movie. This is a common and accepted way of expressing great value in space opera these days.

Mass-to-energy conversion makes this sort of thing difficult (unless the unobtanium is a key component in the process). Energy-to-mass conversion essentially makes anything dirt cheap. At that point, your treasure is going to be works of rare art, especially if you can verify the authenticity of original works. (While AIs might be able to churn out amazing art at astounding speed, the assumption here is that the original creations of idolized artists will still command great value, though potentially only to eccentric collectors.)

There's also the secrets-of-ancient-antiquity version, whether that's the knowledge of a lost (often but not always Golden) age (like the STCs of the Warhammer 40k universe), or dead alien civilizations.

While gemstones can still work (especially if you have a magical tech that can distinguish between natural and artificial) it takes more work. Keep in mind that diamonds are worth as much as they are today in the real world due to the bulk of the natural supply being dribbled out slowly by the principle mine-owners. They're actually quite common, just not in circulation.

Finally, while there's lots of gold and other precious metals potentially floating about, if you're talking about a galaxy-spanning civilization(s) with a population measured in the vigintillions, and most especially if there are practical applications for these metals (conducting electricity is still a thing), demand could still drive up price. This can be especially true if something is disrupting mining efforts; local pirate activity or warfare could drive up the price locally, double so if your FTL travel isn't instantaneous.

I've been casting about for other ideas of what future treasure might look like. If anyone has other suggestions, please feel free to share in the comments.