Worthless Yellow Rocks: Oh, yeah. Starting with gold itself, whose rarity went the way of the dodo shortly after asteroid mining was developed, and whose price is of a similar order to Earth’s prices for iron, if not actually less. (Which is to say, if it didn’t wear down so fast, you could pave streets with the stuff.) Iron, meanwhile, is so ridiculously cheap that most mining companies will let you have it for the cost of hauling the damn stuff away – it’s basically an overly plentiful byproduct of refining metals that are actually worth something.
In any case, this makes the gAu, along with other precious metal currencies like the gAg and the gPt, float very low indeed on the Empire’s currency exchanges and those of the other Core Markets. If you want them to be worth much, it’s time to find some Emerging Market deep in the Periphery, preferably worldbound.
Diamonds, if anything, have suffered even more. They’re trivial to assemble for even the most primitive carbon organizer, to the point where diamondoids are common industrial materials. Worse, enough planets have used carbon-organizers for industrial-scale carbon sequestration that there are entire bloody mountains of bar diamond lying around in places.
In general, advanced materials technologies are really hard on simple valuable stuff. Stuff with complexity included, now, that’s a different matter…
Is there anything like Star Trek Latinum”, some material that is still usable as physical money: divisible, verifiable, portable, and rare, even in the face of asteroid mining and nanofabs?
Not really, in the big picture. There are some exotic matter variants or valuable-yet-obscure-and-hard-to-duplicate-with-natural-variation biologicals that might come close on the basis of intrinsic properties that are valued, but they lack universality; they tend to be more “things used more often for barter than others” rather than any kind of currency.
The modern physical esteyn gets the necessary properties by being a physical token for a cryptographic signature verifiable by the Imperial Banking & Credit Weave, and the exval has also followed that model. So, with variations, have just about all of the Core and First Tier Markets, because that’s what they have that works.