Fulgurite
(c) 2002 Andrew Alden, licensed to About.com, Inc. (fair use policy)
When a bolt of lightning strikes loose sand, the electric charge vaporizes a thin wormhole and melts the zone around it, creating an instant froth of natural glass. These tubes—fulgurites—can be a meter long or more, but they're fragile, and what you see in rock shops is usually a piece like this, 4 centimeters long and as light as pumice. Sometimes a fulgurite forms in solid rock. This fulgurite specimen came from the Sahara Desert in Morocco. See a gallery of fulgurite pictures here and an article about scientific studies of fulgurites here.
Fulgurite is scientific Latin for "lightning stone."
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