The green stone you’re referring to is called serpentine (sometimes “green marble of Prato”) and was used extensively in Tuscany (and elsewhere). To understand why we see so much of it in medieval and early modern Tuscan architecture it helps to look further back: colored marbles (or other stones, although for our purposes “marble” will suffice) were commonly used throughout antiquity all over Italy. When builders in the twelfth, thirteenth, and fourteenth centuries—what we often call Romanesque and Gothic—wanted to call to mind “ancient” architecture one of the ways they did so was through the use of colored marbles. Serpentine, or green marble, is just one of locally available colored marbles that builders made extensive use of (dark grey pietra serena is another that you see very often).

Colored marbles were associated with the splendor of antiquity, thanks to their survival in large- and small-scale works—e.g., buildings and tombs—and thus using them in the Middle Ages was a way to make something visually resplendent while also referencing the glory of antiquity (the resurgence in mosaics during this period, particularly in Roman churches, is an example of a related phenomenon).

This use of colored marbles in the eleventh–thirteenth centuries as a means of referencing antiquity then influenced subsequent architects in later centuries, who occasionally considered Romanesque structures to be “ancient.” The most famous example of this is the Florence Baptistry, which—in its current incarnation—was built in the eleventh/twelfth centuries but was believed to be “ancient” by Florentines just a few hundred years later. The Florence chronicler Giovanni Villani, writing in the early fourteenth century in his La Nuova Cronica, claimed it to have been a Temple to Mars that was later reconsecrated to St John the Baptist and turned into a baptistry. There’s some valid debate over whether Florentines really thought this structure was from antiquity (and there is evidence it was built atop earlier structures, but you could say that about nearly any building in Italy), but from a stylistic and cultural standpoint it served as a model from ”antiquity,” and thus influenced subsequent structures that attempted to look ancient.

This reliance on Romanesque sources as “ancient” models is part of the reason why, e.g., Alberti’s façade of Santa Maria Novella looks the way it does: i.e., a certain classicism (in its proportions and reliance on mathematical relations) filtered through the lens of Romanesque vocabulary (which itself was referring back to antiquity).

As an aside, one of the best ways to see the great variety of colored stones used from antiquity through the Renaissance is simply by looking down in churches—the flooring in many medieval and Renaissance churches is often a great mix of colored stones, in many cases taken directly from ancient sources.

I’m drawing on a lot of my own notes from teaching here, but the literature on Tuscan/Italian architecture is vast. Peter Murray’s The architecture of the Italian Renaissance (1986) is a classic, short primer on the subject. John White, Art and Architecture in Italy: 1250-1400 (1966/rev. ed 1993) is similarly classic, although much more extensive (a bit dry though). Diana Norman’s Siena, Florence, and Padua: Art, Society, and Religion 1280-1400 (two volumes) is an excellent, very readable source on the period.

Hey man, awesome explanation, and very interesting.

I did study Roman archaeology as part of my uni studies, and I do of course know about the use of pigments on marble, both building and statues but I did not associate “colores marbles” with antiquity. Is there any particular Roman or Greek example preserved today that you can think of?