Habet quisque supra domum suum Jovis barbam, went one of the lesser-known edicts of Charlemagne, the Holy Roman Emperor: "Let each one have above his house the beard of Jupiter." The reference was to a herb called "house-leek" - or "Jupiter's beard" as it was known - which, grown on the roof-top of a house, was believed to ward off lightning, fever and evil spirits.
The relevance of Jupiter goes back to Roman times when this chief god was believed to be the fons et origo of lightning and the deadly "thunderbolt". In painting and in sculpture, he was depicted, seated on a throne, clutching the thunderbolts, which looked like little dumbbells, in his hands.
It was said that Jupiter, when displeased, would hurl these missiles at offending parties, and such stories were confirmed when people found strange objects in the ground which closely resembled what a thunderbolt must look like.
The objects concerned were small and glassy, and pale green to black in colour. Nowadays we call them tektites, and they are found in relatively large numbers in a few well defined regions of the world.
They have a very unusual chemical composition, high in silica, and are totally unlike any known terrestrial volcanic glasses, or any known material of meteoritic origin. They appear, from dating techniques and other evidence, to have fallen from the sky in geologically fairly recent times.
Tektites occur in distinct strewn fields, and four separate "arrival events" have been identified. The oldest, in the vicinity of Texas, occurred some 35 million years ago; there are other tektite zones in central Europe and on the Ivory Coast in Africa, and the most recent shower occurred a mere 700,000 years ago in Australasia. Vast numbers of them have been collected in each of these localities, in a wide variety of shapes and sizes.
The most obvious explanation was that tektites were of extraterrestrial origin, particularly since some contain bubbles of vapour at a very low atmospheric pressure, equivalent to that some 18 miles above the Earth. Perhaps they came from ancient volcanic eruptions on the moon? - but analysis of lunar rock brought back by the Apollo missions ruled this out.
Tektites remain one of the great mysteries of geology. A widely held view is that they are fragments of terrestrial rock, liquefied and blasted from the ground by the impact of a meteor, to sail high into the atmosphere and fall to Earth again, the product of a giant, cosmic, meteoric splash. But perhaps the most picturesque explanation is that implied by the Chinese word for tektite: lei-gong-shih means "stools of thunder gods".