Fulgurite

Although most people think of glass as a man-made material, it is found in many forms in the natural world. Glass is formed by volcanoes spewing out molten rock, by lightning striking sand in deserts and on beaches, and by meteorites colliding with the earth. These materials might not look like the transparent, industrial soda-lime glass that we are used to in our windows and lightbulbs, but they are materially related to the man-made glasses that we use every day.

Fulgurites are natural tubes or clumps of sintered, vitrified, and fused soil, sand, rock, organic debris and other sediments that form when lightning discharges into silica-rich ground. The lightning instantly melts materials in its path at temperatures exceeding 1800°C. Fulgurites are therefore known as petrified lightning, and the name comes from the Latin fulgur for thunderbolt. Fulgurites take the form of Lichtenberg figures, which are the fern-like, branching patterns that form on the surface of insulators (wood, skin etc.) when they are hit by high-voltage discharges like lightning. They are often hollow (the lightning is thought to vaporise the sand in the middle) with a rough texture from sand particles adhering to the surface of the molten glass. They are very fragile, so they are rarely excavated intact. This small piece of a fulgurite is from Algeria.