Quick round up of Starry Sept canon, let me know if I've missed anything:
Canon
Appearance:
- It has "black marble walls and arched windows", visible from the exterior, as seen from a bridge near the Quill and Tankard. (Prologue, FfC)
- Also visible from this vantage point are the manses of the pious "clustered like children gathered round the feet of an old dowager". (Prologue, FfC)
- It has "dark marble halls" (WoIaF, The Reach: Oldtown).
- It is counted among the three "great monuments" of Oldtown, along with the Citadel and Hightower. (WoIaF, The Reach: Oldtown).
- The splendour of the castle sept at Highgarden is "rivaled only by that of the Great Sept of Baelor in King's Landing and the Starry Sept of Oldtown." (WoIaF, The Reach: Highgarden).
Significance:
- The Starry Sept has been the seat of the High Septon for a thousand years before Aegon's Conquest. (Prologue, FfC)
- The official start of Aegon's rule is from when he was crowned and anointed in the Starry Sept. (WoIaF, The Reach: Oldtown).
- Lord Tristan Hightower built the Starry Sept in honour of Septon Robeson, who raised him after his father, Lord Damon "the Devout" died of illness. Lord Damon was the first Hightower to accept the faith, and built the first sept in Oldtown as well as six others across his realm. (WoIaF, The Reach: Oldtown).
Interpretations
Size:
- The only clear indicator of size is that in the FfC prologue where Pate describes the manses of the pious as being clustered at its feet. Surely, this would make the sept quite large.
- The quote comparing it to HG's sept and Baelor's only compares
splendour and does not imply it is larger or smaller than either.
Color:
- The FfC prologue suggests that the sept is noticeably black from a great distance, meaning it is likely at the very least,
mostly black.
- WoIaF indicates it also has dark marble halls, meaning at least
some of the interiors are also black marble.
Conclusion
The Starry Sept should be a noticeable landmark on the skyline of Oldtown. It should be significantly taller than a manse. Its size should be a reflection of the Hightower's wealth at the time it was built, which was considerable. It is made of, or at least clad in, black marble, on the outside and at least partly on the inside. While it would be difficult to import all the stone necessary, its location at the mouth of a major river would have made transport from an upriver quarry significantly easier. I believe that no expense would have been spared, and that it was likely built as a proclamation of the Faith's new foothold in the city. There is nothing which would lead me to believe that a source of black marble could not possibly form in the area around Oldtown (or another stone with a similar appearance, see
Ashford Black Marble,
Noir Belge, and
Kilkenny Marble).