The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]