This is an outstanding question and would take more than one book to even rudimentarily answer. That is why I would like to provide just a few bits and pieces h

This is an outstanding question and would take more than one book to even rudimentarily answer. That is why I would like to provide just a few bits and pieces here: There are, of course, some ancient monuments that are still in use today (the Pantheon, the Mausoleum of Hadrian = Castel Sant'Angelo, several amphitheatres). Many architectural elements were reused in later buildings, especially ancient columns in church buildings. But most of reusable material was indeed recycled: Marble was burnt into chalk. Gold, silver and bronze was recast (very few ancient bronze sculptures survived). In contrast, quite a lot of engraved gems of imperial provenance have come down to us; although their whereabouts during the Dark Ages are unknown, it can be assumed that they moved from one royal or ecclesiastical treasury to the next. It is also worth noting that in the long run, much of the precious metal probably ended up in the Far East (China, Southeast Asia and India) as a medium of exchange for the luxury goods that reached Europe via the Silk Road.