Fulgurite

Fulgurite is a natural glass formed when lightning fuses sand or rock, producing the amorphous silica glass lechatelierite (SiO₂). Fulgurites are natural hollow glass tubes formed when lightning strikes sand, soil, or rock. A single lightning bolt delivers temperatures of approximately 30,000°C (54,000°F), roughly five times the surface temperature of the sun. This instantaneous heat fuses silica grains into glass in a fraction of a second.

The resulting tube follows the lightning's path into the ground, often branching like a root system. Sand fulgurites have rough, sandy exteriors where partially melted grains are sintered together, but their interiors are smooth, glassy lechatelierite (amorphous silica). Rock fulgurites form on exposed rock surfaces as thin glassy crusts.

Fulgurites range from a few centimeters to several meters in length. The longest documented specimen exceeded 5 meters. They're extremely fragile, especially the thinner branches, which is why complete specimens are rare. Each fulgurite is a unique record of an individual lightning strike, preserving the exact path the electrical discharge took into the earth.

Fulgurites are identified by their hollow, tubular form with a glassy interior and rough, sand-coated exterior. They're typically irregularly shaped, often branching. The interior glass is usually clear to gray and has a vitreous luster. The exterior is rough and sandy.

Distinguish from tubes created by plant roots (smoother, no glass lining), insect tunnels (organic, no glass), and manufactured glass tubes (uniform wall thickness, no sand coating). Break a small fragment if in doubt: fulgurite glass will show conchoidal fracture.