Fulgurites ("Petrified lightnings")

Fulgurites 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 can therefore be referred to as petrified lightning. 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. 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, or other sediments.