IT IS NOT AN AGATE!

Many rock and mineral collections contain specimens of “turritella agate”; however, the specimens are neither agates nor do they contain the snail Turritella!  Agates are banded (typical such as Lake Superior types) or included chalcedony (many moss agates).  Although “turritella agates” are siliceous, they are neither banded nor included.  Most specimens of “turritella agates” found “for sale” at rock and mineral stores most likely were collected from southwestern Wyoming in the Green River Basin at a locality termed the Delaney Rim.  The Green River Basin was the site of a large freshwater lake during the Eocene Period of the Tertiary (~34 Ma—60 Ma) termed Lake Gosiute.  Related to Lake Gosiute is Lake Uinta that formed in northwestern Colorado (Piceance Basin) and northeastern Utah (Uinta Basin), and Fossil Lake (Fossil Basin) immediately to the west.  The climate around the lakes during the Eocene was tropical to subtropical and the waters teamed with life.  For example, the famous fossil fish localities near Kemmerer, Wyoming, are located in the Green River Formation exposed in the Fossil Basin.  On the lake shores palm trees grew and flamingos waded in the shallow water.  Among invertebrates, the lakes contained millions of snails now assigned to Elimia tenera.  The snails formally were assigned to the genus Goniobasis. Turritella is a well know marine snail (and also differs in external markings from Goniobasis or Elimia).  However, one should be aware that some popular web sites still note such misinterpretations: “Turritella agate is formed from silicified fossil Turritella shells. Turritella are spiral marine gastropods having elongated, spiral shells composed of many whorls”.   And, most rock and mineral stores still label their specimens as ‘turritella agate”!  For a more complete description of the naming problem I refer the reader to a published paper in Rocks and Minerals (Allmon, 2009).

REFERENCES CITED

Allmon, W. D., 2009, The Natural (and not so Natural) History of “Turritella Agate”: Rocks and Minerals, v.84, no. 2, p. 160-165.

mike 

Image from Wikipedia Commons.