In one of those inconsistencies of the rock world, though it is not banded or translucent, this material is known as Turritella Agate.
Although people have called this material Turritella Agate for several decades, the name is actually incorrect. It was mistakenly named after a genus of fossil snails that are very similar to the shells in the agate. The proper name of the snails is "Elimia tenera," a member of the Pleuroceridae family.
These snails are characterized by turreted, cone shaped shells exhibiting long slender whorls with spiral ribbing. The shells of Elimia are distinguishable from real Turritella by being generally shorter and wider. Ancestors of contemporary Turritella and Elimia have left countless shells in Earth's oceans for hundreds of millions of years, all the way back to the Lower Permian Period of the Paleozoic Era.
This semi precious fossil gemstone material Turritella Agate is from the early and middle parts of the Eocene Epoch, between around 53 and 42 million years ago. The spiral-shaped shells accumulated in the sediments of a shallow inland sea in an area that we now know as Wyoming. A few lenses of snail-bearing sediment, in what is today known as the Green River Formation, were then agatized by the deposition of fine-grained silica into the cavities of the shells and the voids between them.
Turritella fossil cabochons are about a 7 hardness, wear well and take a good polish, and Turritella Agate cabochons are very unique and interesting!