So, in one of my worlds, the power system revolves around Branded, people who carry a magical Brand, which grants them a specific power. Each Brand is unique (though some seem similar at first glance), and is passed from person to person down the centuries. Some Branded can also create relics with a more limited version of their power, though some can do this more easily than others. Being a Branded affects your physiology and lifespan in other ways, though weather for good or ill varies.

One of the more famous Branded was the Adamant Empress, who's Brand allowed her to both create and shape diamond in large quantities. By the end of her very long life, she was building giant bridges and hundred-mile walls out of the stuff. Furthermore, her final battle, and the energy release of her death, created veins of diamond underground so extensive as to be functionally infinite.

Between scavenging and mining, bulk raw diamond is roughly as expensive as iron ingots, though of course the diamond is much harder to work with.

Unless you have one of the rare-but-not-super-rare magic relics that the Empress left behind, which allow diamond to be liquefied, purified, stained, and blown like glass, while remaining diamond-hard as a finished product.

Prior to the Adamant Empress, the technology and society was roughly analogous to the High Middle Ages in Europe, though between her hundred-year reign and the hundred-year span after her death, it has mutated into something quite a bit weirder.

In this kind of tech level, what practical use would the existence of cheap bulk diamond be? Diamond would, apparently, be pretty bad for armor and good-but-niche for certain kinds of weapons, but there are plenty of other uses. Optics, hyper-sharp blades, and all sorts of other interesting uses would exist. Furthermore, the Adamant-infused relics, with the ability to fine-tune the composition and structure of diamond so effectively, would we revolutionary today, and I am sure it would have all sorts of insane uses in a less advanced setting.

Anyone have thoughts on specifics?