Not truly a mineral or a fossil, fulgurite is sometimes referred to as petrified lightning. The word comes from Latin (fulgur = lightning), and is amorphous “lechatelierite” silica glass. When lightning or electricity strikes a conductive surface, the temperature can skyrocket to at least 2,950F and melt surrounding silica or quartz leaving behind a glass tube. Lechatelierite can also be formed when a meteor impacts the ground, or during volcanic explosions.
They vary in shape, size, and color, and can be very fragile. Larger pieces have been recorded between 13-30 feet in length. Their shape mimics the path the electrical charge took.
Central Dakota Gem & Mineral Society 