ICE CRYSTALS
Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust." It is reported as "IC" in an observation and on the METAR.
A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates:
