Dragons? Dragons.

To start, let's get out of the way the list of things we know, 100%, about treasure.

Treasure is the primary currency used in Sorneith.

It is a coin-based currency with a golden color and metallic sheen.

It has existed at least as far back as the Sandsurge's creation.

It seems to only posses a single face-value coin.

It is a currency used primarily, if not exclusively, by dragons.

...And that's really about it.

While we've heard about the origin of many things on Sorneith, there's no mention of where treasure is minted, how it's circulated, or even why it was chosen as a currency to begin with. You would think this would be fairly important information, given that all eleven flights are actively at war with each other. Not only would controlling the continent's supply of treasure be a very noteworthy advantage for a flight logistically, but having Sorneith adopt that currency would have a lot of cultural implications as well.

It could be possible that all eleven flights mint their own currency, but again, the chances of these all being the same is fairly unlikely. This implies to me, at least, that treasure is most likely created by some kind of unaffiliated third party.

It's known that there are regions in Sorneith outside the deitiy's influence. It is possible that within one of these areas is where treasure is minted, in much the same way that couriers (as in, the NPCs) are organized through an off-screen organization. However, and much more interesting conceptually, is the idea that treasure is not being minted at all.

This isn't entirely unheard of. Many monetary systems in real life will often select currency that is difficult or impossible to create in order to control inflation and counterfeit -- similarly, if the mints responsible for making coinage were destroyed, then that would also mean nothing new is entering circulation.

Sorneith notably lacks any mention of actually making currency and does not seem to have an active banking system, not even with economically-focused flights like Fire and Lightning. Not to mention that, in a setting where individuals may range from 3" to 30' tall and where the cost of items ranges into the tens of thousands, that only having a single face-value for currency is something you'd only commit to if you were crazy… or if you didn't have the choice.

This is where I pull out my tin-foil hat, so bear with me.

For those of you interested in games, you might know that in the Fallout series, the post-apocalyptic setting's main currency is bottlecaps. They're quite numerous due to their commonality before the war, but the technology required to make more is long since lost. For this reason, they've been adopted by the residents of the west coast as currency to trade for goods and services. And while they originally were only used in one location, the merchants travelling across the continent took that currency with them, which has slowly caused most of the setting to also adopt bottlecaps in their monetary systems.

Sorneith, coincidentally, is also a setting that takes place after a large event wiped out all previous inhabitants -- ones who had much more advanced technology. It is also a setting with many warring factions who all inexplicably use the same monetary system and currency, one which is never mentioned to have been created by any living group and is most prevalent mechanically with a group of merchants who are known to move their business all around Sorneith.

I think Sorneith is in a very similar situation to Fallout's wastelands. The magi of the Second Era were an incredibly advanced civilization whose presence permeated every square inch of the continent, leaving behind not just the ruins of Light, but countless other kingdoms and countries. Meanwhile, the dragons were a young species with no developed culture or technology of their own, who have been at constant war with each other since their creation. They would not have had the time or inclination to create a unified monetary system.

The economy of Sorneith likely began with bartering. Light, being situated in one of the largest Second Era ruins, was probably the first flight to adopt a currency based system. To this day they still utilize magi relics as currency for their flight's festival, and likewise turned to the old coinage scattered across the Sunbeam Ruins as a convenient alternative for trade. As merchants and dragons travelled, they took this habit with them, and over time the majority of dragon society came to view "treasure" as the standard unit of currency.

But that's just a theory...a dragon theory.