What is Turritella Agate?
Turritella Agate is the popular name used for a brown, translucent, fossiliferous agate
found in the Green River Formation of Wyoming.
It is very easy to recognize because it contains large fossil snails that stand out in a white-to-tan color
that contrasts with the brownish agate.
This organic gem material was incorrectly named
decades ago when the christener thought that the spectacular spiral-shaped gastropod (snail)
fossils entombed within the stone were members of the marine
Turritella genus. That was a mistake. Instead, the fossils are of the freshwater snail,
Elimia tenera, a member of the Pleuroteridae family.
Before the correct name was realized and widely published, the gem material became quite popular and the name
“Turritella” went wild in lapidary magazines, gem,
mineral, and fossil books, catalogs, and exhibits. Today it is typically seen without corrective note in all
of those sources, along with websites, online auctions, and computer software. Only a fraction of the people who
have collected the material, cut it into cabochons, sold it, bought it, or worn it in jewelry have any knowledge that
Elimia is a more appropriate name.
It is possible that the misnamed Turritella is the best-known fossil from the Green River Formation.
How did Turritella Agate Form?
About 50 million years ago, during the Eocene epoch, the young Rocky Mountains were almost finished growing, and the
landscape of what is now parts of Colorado, Utah and Wyoming consisted of rugged mountains separated by broad
intermountain basins. Rains falling on the slopes of these mountains ran off of the land and collected into streams
that carried sand, silt, mud, and dissolved materials down into the lakes that occupied the intermountain basins.
Over time, these sediments began filling the lakes, and many types of fossils were preserved within them.
Abundant plants and algae grew on the margins of these lakes, providing a perfect habitat and food source for
Elimia tenera, the freshwater snail. When the snails died, their shells sank to the bottom of the lake. The snails were
so prolific that entire lenses of sediment were composed almost entirely of their shells.
After these layers were
buried, groundwater moved through the sediments. Small amounts of microcrystalline
silica that were dissolved in the groundwater began to precipitate, possibly in the form of a gel, within the cavities of the snail shells and the empty
spaces between them. Over time, the entire mass of fossils was silicified, forming the brown fossiliferous agate
(also known as chalcedony) that we know today as Turritella agate.
The Green River Formation is one of the best-known rock units
in the world for its fossils. Some geologists call it a "lagerstätte," a name given to a rock unit that is exceptionally
rich in fossils. Spectacular fish,
animal fossils have been found
in the Green River Formation.
Turritella Agate: The Gem Material
For at least the past fifty years, Turritella agate has been prized as a unique and beautiful gem material. When
it is completely silicified, it can be sawn into slabs that are polished and used for bookends, desk sets, clock
faces, and other lapidary craft items.
Many lapidaries mark ovals, circles, squares and other shapes
on the slabs and cut them into cabochons. These make great pendants, belt buckles, bolos, ring stones, and earrings.
Scraps and pieces too small to slab or make other items with can be placed in a rock
tumbler to produce some of the most interesting tumbled stones.
People who see these beautiful projects marvel that so many spectacular fossils are preserved in the rock. They are also
surprised by the cross-sections of the fossils that are visible in great detail on the slabs, cabs, and tumbled stones.
Turritella is one of the most fascinating gem materials and a lesson in snail anatomy.
The rock material that contains the fossil snails ranges from a shale
to a sandstone. Some of it has been silicified into
a dense agate that is free of voids and serves as an excellent gem material. However, most of the material is only somewhat
silicified, or unsilicified.
When purchasing or collecting material for lapidary work, it must be carefully inspected
to be sure that it is fully silicifed and that the fossils are firmly cemented into the rock. Some of the vendors who sell it do not know the qualities that are needed for lapidary work. Incompletely silicified
material is a waste of money and time. It is frustrating to cut, yields a low-quality product, and doesn't even make nice tumbled stones.
What About Elimia tenera?
If you go into a rock shop or a gem, mineral, and fossil show and ask for “Elimia Agate,” a lot of people will
say that they have never heard of it. But, if you ask for Turritella, almost everyone around you will know what it is.
The incorrect name is that well-entrenched in the lapidary trade.
If you want to learn more about Elimia tenera and the best name for Turritella Agate, we recommend a visit to the
Museum of the Earth website,
which is authored by staff paleontologists of the Paleontological Research Institution - people who know what they are
talking about when it comes to fossils. Their article on Turritella Agate was authored by Dr. Warren D. Allmon, Director
of the Museum.
Contributor: Hobart King
Find it on Geology.com
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|A close-up photo of a sawn surface of Turritella Agate showing numerous spiral-shaped snail shells. This type of view shows how the internal cavities of the shells and the voids between the shells have all been infilled with the translucent-to-transparent agate. This view is about two inches across.
|A cabochon cut from Turritella Agate that features one of the spiral-shaped snail shells. This cab is about 1 1/2 by 1 inch in size.
|A slab of Turritella Agate about six inches in width and 3/8 inch thick. Slabs like this are used to cut cabochons. The lapidary selects a nice scene in the slab, saws a rough outline, and grinds the material into a cab.
|Another close-up photo of Turritella Agate. The width of this view is about two inches.