Ice Crystal
Hexagonal columnar prism, the most basic form of a snow crystal. The six-fold symmetry is the consequence of the shape of water bonds; the facets managed to form so clearly due to very slow crystal growth.
I dug this chunk out of a clay-rich bank of a creek during a period of borderline freezing (±3°C) temperatures.
Coincidentally, this was the same time Matt at Agile* Geoscience was finding ice cups in his bird bath on the opposite side of the country. It was a good week for ice!