Construction of the Sixth Empire
As noted by Dalt, the Sixth Empire used a gray stone, basalt type, or gray stone of a porous type that has not been identified. The gray stone is not only exceptionally hard for basalt, but also seems to have a lot of resonance with magic (according to Sebastian) allowing it to be easily magiced. Decay rate for persistent magics appear to be a massive 10:1 scale. (IE: a 1 minute spell will last over 10 minutes, and sometimes longer, when cast upon the stone) The stones are tightly fit together, with a gray colored mortar that matches the stone quite well. The stone and mortar are then polished so that there appears to be no gap or mortar, and then either painted, faced with plaster, or faced with marble.
This means that Sixth Empire construction is built to last. As ruins have been found, some sort of disaster capable of destroying buildings built in such a manner must have occurred. Dalt knows that the stone is some of the hardest he has seen outside of pure granite, and the cuts are extremely precise and regular.
Road construction uses hexagonal stones of uniform size. Although many roads have been destroyed, it appears that in many places the stones were pulled up, possibly used as building supplies by survivors of whatever disaster destroyed the Sixth Empire.
The Sixth Empire also used the arch, vaults, column reinforcement, and other ways to make their construction outlast what Dalt has seen as far as construction within the Republic. They seemed to build for massive redundancy, although many of the buildings seem to have collapsed for some reason.
Massive piles of stone, which was first believed to have been built by survivors or placed by the Republic appear to have been moved into piles, with the stones numbered and stacked a certain way, by the Sixth Empire.
What this means is currently unknown.