Fulgurites are formed when lightning with a temperature of at least 1,800 °C (3,270 °F) instantaneously melts silica on a conductive surface and fuses 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.
The color varies depending on the composition of the sand they formed in, ranging from black or tan, to green or a translucent white. The interior is normally very smooth or lined with fine bubbles; the exterior is generally coated with rough sand particles and is porous. They are rootlike in appearance and often show branching or small holes. 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. The fact that fulgurites are abundant in the Saharan Desert shows that lightning was once a frequent occurrence in that region.
Fulgurites are also popular among hobbyists and collectors of natural specimens.