chalcedony | Show-Me Rockhounds Kansas City

Special guest article from Show-Me Rockhounds club members Dan and Connie Snow

Group picture of Fairburn agates – Dan Snow

Fairburn agates are a form of microcrystalline chalcedony, 100% silicon dioxide with a hardness of 6 ½ to 7 on the Mohs scale. They are also called fortification agates because of their banding. They were formed approximately 300 million years ago in an ancient limestone bed of an inland sea. To hunt Fairburn agates requires looking at every rock and turning many with a rock pick. It is strictly surface hunting no digging, mining, cracking or breaking rocks. The photos shown are exactly the way the agates were found, with no cutting, polishing or tumbling having been done.

Frog Rock – Dan Snow

Down under agate – Dan Snow

The Conscious Eye – Dan Snow

Fairburn agate – Dan Snow

Water agate – Dan Snow

Finger agate – Dan Snow

Fairburn Agates found by Dan and Connie Snow. Collected from the Fairburn Agate beds of South Dakota and the Oglala National Grasslands in Nebraska.

Article by special guest author David Reed

a long flowing wing-shaped rock appearing to be made of several small tubes, with red and blue colors.

From the collection of David Reed, photo by Stephanie 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.

Photo by Stephanie Reed

Photo by Stephanie Reed

Lots of chalcedony filaments all pointing the same way

Photo by Stephanie Reed

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.

Lechuguilla Chandelier Ballroom photo by Dave Bunnell

long squiggly white directional helictites

Directional Helictites Photo by Dave Bunnell

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.

A round brown chunk of tiny tubes of coral with a white crust on the outside

From the collection of David Reed, photo by Stephanie Reed