Fulgurites (from Latin fulgur 'lightning' and -ite), commonly called "fossilized lightning", are natural tubes, clumps, or masses of sintered, vitrified, or fused soil, sand, rock, organic debris and other sediments that sometimes form when lightning discharges into ground. When composed of silica, fulgurites are classified as a variety of the mineraloid lechatelierite.

When ordinary negative polarity cloud-ground lightning discharges into a grounding substrate, greater than 100 million volts (100 MV) of potential difference may be bridged. Such current may propagate into silica-rich quartzose sand, mixed soil, clay, or other sediments, rapidly vaporizing and melting resistant materials within such a common dissipation regime. This results in the formation of generally hollow and/or vesicular, branching assemblages of glassy tubes, crusts, and clumped masses. Fulgurites have no fixed composition because their chemical composition is determined by the physical and chemical properties of whatever material is being struck by lightning.

Fulgurites are structurally similar 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. The primary SiO₂ phase in common tube fulgurites is lechatelierite, an amorphous silica glass. Many fulgurites show some evidence of crystallization: in addition to glasses, many are partially protocrystalline or microcrystalline. Because fulgurites are generally amorphous in structure, fulgurites are classified as mineraloids. Peak temperatures within a lightning channel exceed 30,000 K, with sufficient pressure to produce planar deformation features in SiO₂, a kind of polymorphism. This is also known colloquially as shocked quartz.

Material properties (size, color, texture) of fulgurites vary widely, depending on the size of the lightning bolt and the composition and moisture content of the surface struck by lightning. Most natural fulgurites fall on a spectrum from white to black. Iron is a common impurity that can result in a deep brownish-green coloration. Lechatelierite similar to fulgurites can also be produced via controlled (or uncontrolled) arcing of artificial electricity into a medium. Downed high voltage power lines have produced brightly colored lechatelierites, due to the incorporation of copper or other materials from the power lines. Brightly colored lechatelierites resembling fulgurites are usually synthetic and reflect the incorporation of synthetic materials. However, lightning can strike man-made objects, resulting in colored fulgurites.

The interior of Type I (sand) fulgurites normally is smooth or lined with fine bubbles, while their exteriors are coated with rough sedimentary particles or small rocks. Other types of fulgurites are usually vesicular, and may lack an open central tube; their exteriors can be porous or smooth. Branching fulgurites display fractal-like self-similarity and structural scale invariance as a macroscopic or microscopic network of root-like branches, and can display this texture without central channels or obvious divergence from morphology of context or target (e.g. sheet-like melt, rock fulgurites). Fulgurites are usually fragile, making the field collection of large specimens difficult.

Fulgurites can exceed 20 centimeters in diameter and can penetrate deep into the subsoil, sometimes occurring as far as 15 m (49 ft) below the surface that was struck, although they may also form directly on a sedimentary surface. One of the longest fulgurites to have been found in modern times was a little over 4.9 m (16 ft) in length, found in northern Florida. The Yale University Peabody Museum of Natural History displays one of the longest known preserved fulgurites, approximately 4 m (13 ft) in length. Charles Darwin in The Voyage of the Beagle recorded that tubes such as these found in Drigg, Cumberland, UK reached a length of 9.1 m (30 ft). Fulgurites at Winans Lake, Livingston County, Michigan, extended discontinuously throughout a 30 m range and arguably include the largest reported fulgurite mass ever recovered and described: its largest section extending approximately 16 ft (4.88 m) in length by 1 ft in diameter (30 cm).

Fulgurites have been classified into five types related to the type of sediment in which the fulgurite formed, as follows: