Diamond Dust

A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates.

Diamond dust (ice needles, frost in the air, frost mist) A type of precipitation composed of slowly falling, very small, unbranched crystals of ice which often seem to float in the air. It may fall from a high cloud or from a cloudless sky. It usually occurs under frosty weather conditions (under very low air temperatures).