Often known as petrified lightning, fulgurites are lechatelierites produced by lighting striking sand or silica rock. The name comes from the Latin fulgur, meaning 'lightning', and the postfix -ite, which in this case is quasi-Latin meaning 'rock' or 'mineral'.

Sand fulgurites take the form of irregular 'glass' tubes, often branched or forked. The outsides are generally grey or sand-coloured, somewhat pitted and lumpy, and look rather like coral. The inside is glass-like, although it may be either smooth or pitted and 'bubbly'. They can be found in deserts, on beaches, and in sand dunes, where wind erosion may leave them sticking up above the surface. The size is highly variable; they may be a few inches or many feet long, and up to a few inches in diameter. The walls of the tube are usually very very thin (one source claims 1/20th of an inch, although they can be thicker), and larger fulgurites are rarely exhumed in one piece.

Rock fulgurites are often limited to the surface of the rock, although they may penetrate deeper. their shape is highly irregular; in form they can look like obsidian, although they are often rough and ragged, and in some cases look almost sponge-like.

Fulgurites are near pure silica (SiO2), fused (not crystallized) by a direct lightning strike, superheating the sand or rock. The melting point of silica is 2950 F; the temperature must be at least this hot, and is usually much hotter.

It may be possible that the gasses escaping from the fulgurite just after a lightning strike sometimes ignite, creating ball lightning. This would not explain all reported cases of ball lightning, but seems to be a good theory as to the cause of at least some cases.

You can see sand fulgurite at www.amnh.org/learn/musings/SP01/mb.htm
Rock fulgurites can be seen at http://www.minresco.com/fulgurites/caca.htm

More can be read about the ball lightning theory at: