fulgurite

A tubular structure of vitrified silica, root-shaped, that forms in sandy, dry terrain as a consequence of a lightning strike hitting the ground. The high temperatures - of up to 4000°C - caused by the electrical discharge on its underground path melt the grains of sand, forming a narrow conduit of glassy material, which frequently presents branches. The evaporation of groundwater, which also takes place underground, and the subsequent rapid cooling, contribute to the formation of the hollow inside the fulgurite. These "lightning tubes" present diameters ranging from just a few millimeters to 5 centimeters, and their length can sometimes exceed a meter. Depending on the composition of the sand, it presents different colors. The word has its etymological origin in the Latin term fulgur, meaning lightning or flash. A fulgurite should not be confused with a ceraunia, which is the name given to a lightning stone or point.