diamond dust

Diamond dust is the name commonly used to refer to a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation. It is most commonly observed in Antarctica and the Arctic, but it can occur anywhere with a temperature well below freezing. In Polar regions diamond dust may continue for several days without interruption.
See more at Wikipedia.org...

Noun
1. small crystals of ice
(synonym) ice crystal, snow mist, poudrin, ice needle, frost snow, frost mist
(hypernym) crystal
(part-holonym) precipitation, downfall
(substance-meronym) water, H2O