Ristoro studies a gemstone beside an astronomical instrument.

What sapphire and emerald owe to the heavens

Ristoro d’Arezzo, a thirteenth-century Italian author, completed a description of the composition of the world in the vernacular in 1282. He asks how very different things can come into being from the four elements. Two gemstones give the problem a form that a reader can picture. Put a sapphire and an emerald side by side and their difference is readily apparent. It is harder to say why the same basic materials should produce precisely these two sharply distinguishable bodies.

Four elements and an open question

And we know, and it is evident, that the four elements cannot move of themselves, nor make the sapphire or the emerald or bring forth anything else without the power of heaven, just as flour cannot become bread without water. […]

– Ristoro d’Arezzo, The Composition of the World, 1858, p. 109.[1]

Ristoro directs attention to the capacity for movement. In his account the elements on their own lack the power to set about making such things. He therefore introduces the heavens and their efficacy into the explanation. The image now reaches from a little piece of coloured matter one could turn over in the hand to a cause encompassing the entire visible scene. The distance is part of what makes the explanation so daring.

Why flour enters the comparison

In this connection Ristoro reaches for an experience from the kitchen. Flour alone cannot turn into bread while water is missing. The comparison brings into everyday reach a distinction that could easily seem abstract when applied to the heavens. One can possess a basic material and still depend on something else for a particular result to come about. Readers who grasp the distinction can understand why Ristoro continues his questioning beyond the four elements. The pair of gemstones gives that question two unmistakable colours.