Fulgurite - Mineraloid Lechatelierite - Petrified Lightning - Sahara Desert
With internal glass tube like cavity

Fulgurites (from the Latin fulgur, meaning "lightning") are natural tubes, clumps, or masses of sintered, vitrified, and/or fused soil, sand, rock, organic debris and other sediments that can form when lightning discharges into ground. They are classified as a variety of the mineraloid lechatelierite, although their absolute chemical composition is dependent on the physical and chemical properties of the usually granular-crystalline material providing an electrically and thermally conductive dissipation network for lightning-facilitated energy transfer.

 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

Fulgurites are formed when lightning strikes the ground, fusing and vitrifying mineral grains. Peak temperatures within a lightning channel are known to exceed 30,000 K, with sufficient pressure to produce planar deformation features, or "shock lamellae," in SiO2, a kind of polymorphism (materials science). This is also known colloquially as shocked quartz. "Artificial fulgurites can also be produced when the controlled arcing of electricity into an appropriate medium. Downed high voltage power lines have produced fulgurite-like lechatelierites, occasionally colored by copper from the power lines themselves.

The primary SiO2 phase in common tube fulgurites is lechatelierite, a silica glass also found in impactites, but many other glasses may result from the processes involved with the production of fulgurites given their chemical context. Because their groundmass is generally amorphous in structure, fulgurites are classified as mineraloids.