Petrified lightning is a popular name for fulgurite (Latin fulgure, lightning), which consists of siliceous tubes formed vertically in loose sand by the passage of lightning. When lightning strikes sand a temperature of several thousand degrees Fahrenheit is created and the particles along the central part of the path are volatilized, driven out and fused into a tube with the interior surface glossy and smooth as glass. Such tubes extending to a depth of thirty feet or more have been found, and occasionally they are three or four inches in circumference, although generally they are smaller. As a rule the thickness of the walls is not more than one-thirtieth or one-twentieth of an inch. Sand hills unprotected by vegetation are constantly shifting[,] and not infrequently petrified lightning tubes are left projecting several feet above the surface. Sometimes the tubes are branched and a large number of them together creates the weird effect of a glass forest. Fulgurites are also occasionally produced by lightning running through a wire or cable buried in sand. Tubes resembling natural fulgurites have been produced in the laboratory by discharging electricity of a high voltage through powdered glass or sand of the proper composition.
From A Book About a Thousand Things (1946), by George Stimson