When Lightning Strikes Sand. Fulgurites (Latin, meaning "thunderbolt") Sometimes referred to as petrified lightning, they are natural hollow glass tubes formed by lightning strikes in quartzose sand, silica, or soil. (Wikipedpia)
melonopoly: Mammatus clouds are most often associated with the anvil cloud and severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds.