Onyx can be produced synthetically. Unless there is a requirement that it be NATURAL onyx, simple mineral quartz and a few other ingredients can be used to mass manufacture it, under the right conditions.
Re: vivomancy-- it noted that I did not know what game system he was using, but that SOME systems did use it as a magic school. Since this is a home-brewed scenario, he can homebrew that in too, since the society would naturally favor the emergence of something like it, simply due to economic pressures.
Draignean:
Easy enough-- Collect the old, who have a large reserve of life experiences, or the very young who have a high blood debt, but high vitality-- and instead of simply insanguining them, put them into the vats, and artificially sustain them. When they can no longer be sustained, their corpses become the raw material for the farming labor industry. If you want added horror, say they are alive and conscious through this entire process.
Craft: Flesh Bonsai would certainly be an interesting skill, but it would require a great deal of forethought.
Ah, but the old are filled with the thoughts of their own mortality: A bitter vintage indeed, and if they're kept conscious through the process... Well, that presents its own problems with the flavor. The result would be something potentially sustaining, but also very nearly unpalatable. As mentioned by a later post, perhaps useful as a method of extending the food supply for lower rungs of the necrocracy.
The culinary vampires do certainly collect the old who are interesting, and the culinary vampires are most certainly the ones running the orphanages and debtors prisons, and then you get back into suspended dream states.
Getting the most blood out the population is not the problem the Necrocracy has been dealing with, there has never been a cataclysmically major shortage of blood during their tenure, and thus the focus of their research into blood has been one of pleasurable quality valued high above quantity.
In general, Necrocrat vampires are loathe to create more Necrocrat vampires. It means more people tugging at the pie, trying to get their own piece. It can certainly strengthen your side, particularly if you've got enough space to keep them enslaved, but then you've got to take care of the neonate in the great political machinations created by a court of vampires that have very little better to do with their time than stab each other in the back and indulge in the most succulent pleasures.
It wouldn't matter anyway, since in D&D undead creation is tied directly to the material value of the catalyst. If you start mass producing onyx, your spells will soon need more and more of the stuff. To the point where you're jamming their skulls full of handfuls of the stuff, filling their chest cavities with rocks, and eventually having to grind up massive onyx boulders to bathe the bones in their dust!
Truely woe to the necromancer who thinks he might find a way to improve the efficiency of his craft.
I'm more likely to rule that the gold-piece cost to produce onyx happens to coincidentally match (or even exceed) the GP value of current onyx at the same size. It's kind of a kludge option, but it's workable.
I honestly hadn't considered the material implications of the undead farm labor. Onyx would become the new oil.
I'm sure a society like this one would have no problem raiding and pillaging for onyx. Undead hordes, vampire nobles, and I guess a few human guards as well... perhaps not the best large-scale raiders, however. Besides, if power is tied to the value, won't onyx get more valuable, and hence more powerful, as it gets scarcer?
Alternatively, trade for onyx could be carried out, perhaps by nobles who have learned crafts to impossibly masterful degrees out of boredom. Or by a skilled human class. Naturally, the latter might seek to take their goods and services directly to the buyers, other kingdoms that care about things apart from death and blood, but the heavily restricted roads would prevent them from doing so. Open country, you say? Fifty thousand semi-controlled undead beg to disagree.
I like the GP value system of magic about as much as I like forced alignment changes. You'll still need the same amount of Onyx, regardless of what you pay for it or its value relative to the nearest adjacent economy.
As far as raiding goes, one of the very, very few things the Dracolich forbade his pet vampires was open war between eachother. The lines of the old nations now represent clan territories under the control of various bloodlords, and that's final. No bloodlord is allowed to openly steal or attack the properties of another lord.
Keyword: openly. Onyx raiding might be a job given to a group of Talents that look suspiciously like adventurers.
As far as roads: Roads are great as long as you are openly property to someone. The Necrocracy is strongly Lawful Evil, and they want their society to run like a well oiled machine, one designed to turn a mortal life into a tasty beverage, a useful asset, or an entertaining diversion.
I kinda like the use of Method#1 above though.
If you throw in some kinks-- To use a parcel of land to create artificial gemstones, you need:
1) The spell perk, obviously (House rules, so can be NPC only)
2) A parcel of land who's value is greater than the value of the mineral load you intend to create on it.
3) Refined raw materials to sew into the parcel of land, from which to precipitate the gemstones, who have a high cost value, due to the labor involved in their manufacture/purity.
4) A sum of gold to pay for administration fees, and servicing fee for the geomancer doing the dirty work.
5) Once created, the value of the load of gemstones, minus the costs of the raw material and admin fees, are permanently subtracted from the land parcel's value, in the form of ecological damage. (meaning the land's value permanently drops if you cheese it on the materials or fees paid, which means if you try again later, you cannot produce as many gems, as your land parcel's value is permanently reduced from your mismanagement)
This will invariably create a potentially infinite supply of gemstones, as long as you supply the needed cashflow, without reducing the value of the resulting products. (The resulting products are used industrially, and their demand is tied to that industrial use-- they are consumed by the process that uses them, so the market cannot become saturated. The cost naturally incorporates all prior cost centers, (admin fees, processing fees, wages, etc.), so the synthetic onyx will be more expensive than imported natural stone-- just more available in bulk.
I like this idea rather a lot. I mean, there'd have to be repercussions, but there's a fun one in the setting's wings that would be rather nice to drop in.