A stone that keeps time with the moon

Isidore of Seville (c. 560–636), a scholar and bishop in Visigothic Spain, gathered in his Etymologies the knowledge that clung to the names of things. Among the stones he found one that already bore the moon in its name. The Greek word for moon helped him explain the designation selenite. The entry becomes especially interesting when he moves from the sound of the word to something said to happen inside the stone.

A name directs the eye

Selenites, whose name in Latin means moonstone, because its inner brightness is said to wax and wane with the moon, is formed in Persia.

– Isidore of Seville, Etymologiae, 1911, XVI, 4, 6.^[1]

Its brightness, according to the account, increases and decreases. A person turning a stone in strong light can readily make a bright patch vanish and return. Isidore’s sentence requires a different act of imagination. Here the change follows the course of the moon. The stone might remain in the same place while the strength of its shine alters with the passing days. A reader must put time into what initially looks like the description of an ordinary specimen.

Time enters the description

Isidore signals that he is passing on a report. The familiar rhythm of the visible moon gives his readers a way to picture the process attributed to the stone. That rhythm alone hardly permits a secure identification with a modern mineral species. It does, however, bring out a feature of medieval descriptions that a cabinet of specimens might conceal. What distinguished a stone could include something people said it did over the course of a succession of nights.