In last month’s blog I explained that for many gemstone beads, the country of origin may be difficult to determine. Fossil turritella agate is an exception because it comes exclusively from the Green River Formation in Wyoming, USA.
Turritellas are marine snails (gastropods) with spiral shells. When fossil snails were found in the Green River Formation, they looked like turritellas and were given that name. However, it turned out that the snails were in fact an extinct freshwater variety and were renamed Elimia tenera. By that time, the incorrect name had already become commonly used and the name elimia tenera has never managed to replace the incorrect turritella name.
Turritella fossils are among my favorite fossils to incorporate into my jewelry designs; the fossil snails are creamy to white spirals in a rich brown to almost black matrix. The polish can be uneven on these stones due to the natural variation in the fossils, but that’s part of what makes each one unique.
The following is directly from http://geology.com/gemstones/turritella/:
“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.”