Fulgurites (from the Latin for thunderbolt) are natural hollow carrot-shaped glass tubes formed in sandy soil by lightning strikes. They are up to a couple centimeters in diameter, and can be meters long if they are carefully excavated so as not to break the fragile structure. The interior is normally very smooth or lined with fine bubbles; the exterior is generally coated with rough sand particles. They are rootlike in appearance and often show branching or small holes. Fulgurites occasionally form as glazing on solid rocks, as well.

Fulgurites are notably found high on Mount Thielsen ("the lightning rod of the Cascade Range") where they form a brownish-green glaze on rocks (especially on the final five or ten feet of the summit pinnacle).