Spoilers of course...
Y'shtola already mentioned though in the Tempest that any Ancient remnants on the Source would have long since been obliterated by the constant cycles of Calamities occurring which never happen on Shards (until a Shard is destroyed when it's Rejoined anyway), which is why the ruins of Amaurot still survived intact on the First, so alas the likelihood of finding equivelant Amaurot ruins on the Source.. is almost nil I'm sorry to say.
And you only have to look at Eorzea, where virtually nothing from even the First and Second Astral Eras is known to exist, and even early Sixth Astral Era ruins are few and far between (even the mighty Allagan Empire, the greatest and most technologically advanced civillization the Source has ever seen.... mostly ended up forgotten, before the Seventh Calamity it's name was known only from a ruined cobblestone road in Thanalan and mysterious runestones etched with symbols of The Twelve, and even post Calamity, Allagan remnants that are discovered like the Crystal Tower and Azys Lla are reacted with in genuine surprise.
It is possible that a lot of the fauna/monsters seen in the Source and Shards were once Ancient creations, or at least descended from them (such as griffins), but I don't think that really goes all the way to actual landscapes also being Ancient creations. Crystalline structures seen in the environment seem to be more natural, created simply as a result of aether flowing and colascing (this is why Mor Dhona is almost completely crystallized - successive events there that resulted in massive releases of aether over the past 20 or so years have basically polluted the land with so much aether that has cooled and crystallized, (the Battle of Silvertear Skies, the Calamity). This is also seen in other places around Eorzea where pieces of Dalamud landed after Bahamut blew it to pieces - all that aether released immediately crystallized into weird spiralling and jagged spires, most clearly seen in Eastern Thanalan at the Burning Wall, but also in Eastern La Noscea in Ageless Wyse.
Given the floating islands in the Sea of Clouds like the Diadem float due to overaspected/corrupted wind elemental crystals, the central crystal of the Diadem probably had a similar origin.