Petrified lightning, known as fulgurite, is formed when lightning with a temperature of at least 1,800 °C (3,270 °F) melts silica or other common conductive and semi-conductive minerals and substrates, fusing, vitrifying, oxidizing and reducing mineral grains and organic compounds.
According to Wikipedia
“Fulgurites (from the Latin fulgur, meaning “lightning”) are classified generically as a variety of the mineraloid lechatelierite, although their absolute chemical composition is dependent on the physical and chemical properties of target material affected by the discharge of cloud-ground lightning. They are commonly hollow and/or branching assemblages of glassy, protocrystalline, and heterogeneously-microcrystalline tubes, crusts, slags, vesicular masses, and clusters of refractory materials that often form during the discharge phase of lightning strikes propagating into silica-rich quartzose sand, mixed soil, clay, caliche and other carbonate-rich sediments. Colloquially, they have been referred to as petrified lightning. Fulgurites are homologous to Lichtenberg figures, which are the branching patterns produced on surfaces of insulators during dielectric breakdown by high-voltage discharges, such as lightning.”
The Hollywood film Sweet Home Alabama portrayed sand stricken lightning as clear as glass (see above image), but in reality the end result is actually a fascinating hollow fragile (and fractal) rock as you can see in the gallery below!