How deep does the green of an emerald go?

Girolamo Cardano (1501–1576), an Italian physician and student of nature, examines the emerald in his writings on precious stones. A single word might seem enough to describe it. Someone shown a green stone can give its colour at once. Cardano is interested in the moment when that quick answer begins to feel inadequate. He addresses his reader as if the reader had seen something in the stone that called for a question, and had turned to him for a possible explanation.

Looking into the stone

You ask the cause of the marvel. In this gem there lies a manifold greenness, as if in layers of bark, one hidden beneath another.

– Girolamo Cardano, Historiae lapidum, 1663, p. 522.^[1]

His answer offers a small piece of guidance about where to look. To his eye there are several greens in the stone, with one hidden beneath another. The reader has to move beyond the first visible surface and direct attention into the material. Anyone looking at a translucent body can follow the shift. The eye can attend to its near edge and then to a patch that appears to lie farther back. Both belong to a single object small enough to hold in the hand.

One green behind another

Cardano's observation also touches on the language of description. As soon as he tries to say exactly what holds the eye in an emerald, a word for a colour becomes an account of depth. What lies hidden is immediately behind something visible and shares its hue. That resemblance makes the two hard to separate. A reader who follows the movement of the sentence looks twice at the same stone. What first appeared to be one saturated colour has acquired a foreground and a distance.