Fulgurites are formed when lightning with a temperature of at least 1,800 °C (3,270 °F) melts silica on a conductive surface and fuses mineral grains together; the fulgurite tube is the cooled product. This process occurs over a timespan of around one second, and leaves evidence of the lightning path and its dispersion over the surface or into the earth. Fulgurites can also be produced when the cables of a high voltageelectrical distribution network break, and the wires fall onto a conductive surface beneath, in the presence of loose sand.
The color varies depending on the composition of the sand in which they formed, ranging from black or tan, to green or a translucent white. The interior normally is very smooth or lined with fine bubbles; the exterior generally is coated with rough sand particles and is porous. Fulgurites are rootlike in appearance and often show branching or small holes. Fulgurites formed in sand or loose soil are mechanically fragile, making the field collection of large specimens difficult.
Fulgurites occasionally form as glazed tracks on solid rocks (sometimes referred to as an exogenic fulgurite).
Fulgurites are appreciated by many for their scientific value as permanent tangible evidence of transient lightning strikes. For instance, the fact that fulgurites are abundant in the Saharan Desert demonstrates that lightning once was a frequent occurrence in that region.
Fulgurites also are popular among hobbyists and collectors of natural specimens.