Ice crystal

Ice crystals were a naturally-occurring weather phenomenon. The cloud belt covering the planet Ember had clouds made of ice crystals with tiny electrical charges. Sky fish had evolved to fly through these clouds, but they could only live outside them for a little while. (TV: A Christmas Carol)

In 1979, the Tenth Doctor foiled the Zalphon's invasion of Earth by directing their mothership to Antarctica, where the gaseous-ectoplasmic species would freeze into ice crystals. (PROSE: Bennelong Point)