The Collaborative International Dictionary
Fulgurite \Ful"gu*rite\, n. [L. fulguritus, p. p. of fulgurire to strike with lightning, fr. fulgur lightning: cf. F. fulgurite.] A vitrified sand tube produced by the striking of lightning on sand; a lightning tube; also, the portion of rock surface fused by a lightning discharge.
Wiktionary
n. glass formed by a lightning strike melting sand or other material
Wikipedia
Fulgurites (from the Latin fulgur, meaning " lightning") are classified generically as a variety of the mineraloid lechatelierite, although their absolute chemical composition is dependent on the physical and chemical properties of target material affected by the discharge of cloud-ground lightning. 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, caliche and other carbonate-rich sediments. Colloquially, they have been referred to as petrified lightning. 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.
Usage examples of "fulgurite".
But I was saved by the very thunder that had summoned it, for after a hideous wait there burst from the unseen outside sky one of those frequent mountainward bolts whose aftermath I had noticed here and there as gashes of disturbed earth and fulgurites of various sizes.
All the creatures that had been at vigil with him in the night were gone and about him lay only the strange coral shapes of fulgurite in their scorched furrows fused out of the sand where ball lightning had run upon the ground in the night hissing and stinking of sulphur.