Glass is made by melting sand, and lightning strikes can make it naturally

Window glass begins as something humble: sand. Specifically the silica (silicon dioxide) in quartz sand, which fuses into transparent glass when heated past roughly 1,700 °C. Heat it, melt it, cool it fast enough that the atoms never re-crystallize, and you get the glassy solid we shape into bottles and windows.

Nature has its own glassmaker, and it is far more dramatic. When lightning strikes sandy ground, it dumps an enormous burst of heat into a single channel. According to the U.S. National Park Service, the bolt reaches around 50,000 °F — hotter than the surface of the Sun — and “melts the surrounding sand into dark glass.”

One second, one strike, and the ground keeps a glass fossil of the lightning’s path.

The result is a fulgurite (from the Latin fulgur, “lightning”): a slender, hollow glass tube tracing the bolt’s route underground. The Utah Geological Survey describes them as “natural tubes or crusts of glass formed by the fusion of silica sand or rock from a lightning strike.” The hollow core forms because moisture and air in the soil flash to vapor and blow the molten center open.

The glass itself is called lechatelierite — pure fused silica, the same kind formed by meteorite impacts and volcanic blasts. Fulgurites can run several centimeters wide and stretch meters long, rough on the outside, smooth and glassy within: literally petrified lightning.