Crevasse hoar

From Glossary of Meteorology

crevasse hoar

Ice crystals that form and grow in glacial crevasses and in other cavities where a large cooled space is formed and in which water vapor can accumulate under calm, still conditions; a type of hoarfrost.

They have an origin similar to that of depth hoar; the typical crystal is a hollow cup with one side opening inward and continued in a hexagonal scroll.