Article by special guest author David Reed
This refers to a surface chalcedony formation characterized by groups of chalcedony filaments often intricately woven or connected together, so they resemble the feathers of a wing or flowing hair. They occur most often in the center of a vug or vein of agate, but can also occur in the center of a hollow thunderegg. These formations are usually found in Idaho or Oregon. It describes this type of surface chalcedony formation, regardless of whether the underlying formation is plume agate, tube agate, or moss agate. See below for several close-ups, all from the same specimen.
The tubes in Angelwing Chalcedony seem to follow the direction of flow of the silica-bearing fluid in air within the vug. They may form in similar fashion to the directional helictites (gypsum formations) in Lechugilla Cave (and elsewhere), or they may be directional helictites which were silicified.
Although it looks similar, Angelwing Chalcedony is not the radiating tubes found in fossils of certain coral heads. Angelwing Chalcedony was never alive, but the coral was. During mineralization, the form of the living coral was maintained, but the structure was changed from mostly calcite to mostly silica, and some of the voids were filled. The structure of the fossil is more regular; there was no irregular flow of fluid through a void, as there was with the Angelwing Chalcedony. The fossil specimen below was found eroding out of a Florida riverbed. It was purchased, to avoid diving with the alligators.