[Rumil]

One thing about the 3rd Age fortifications is thar they had been built by the Numenoreans using some mysterious technology or technique.

Orthanc is desrcibed as being of dark stone and very smooth, such that the ents could not gain a hand/tendril hold. To me this suggests some sort of stone like marble, that could be highly polished and almost glassy-smooth. I guess the joints between the blocks must have been very thin and accurate, this could be a product of great craftsmanship, such as ancient South American stone construction that is amazingly precisely put together without recourse to cement and mortar.

The walls of Minas Tirith seem to be much the same except in brilliant white stone, and marble does come in different colours, so this is consistent.