Ammolite - A Star of the Canadian Cretaceous

Dragon-skin ammolite doublet from the K-Zone of the Bearpaw Fm. Author's own, available for setting in jewelry.

As a geologist who’s worked in Calgary how could I not love ammolite - a stunning rainbow-colored gem derived from fossilized ammonite shells? The best material is found only in Alberta, Canada - it’s rare, beautiful, geologically meaningful, and stunning!
There is often confusion between the words ammoLite and ammoNite. AmmoNite is an extinct marine mollusk (~65 million years ago). AmmoLite is a gemstone created from the ammonite shell. Ammolite is only found in the Bearpaw Formation, which extends from Alberta to Saskatchewan in Canada and south to Montana in the US. The best gem-quality ammolite is found along the eastern slopes of the Rocky Mountains in southern Alberta. Most commercial mining takes place along the banks of the St. Mary River, south of Lethbridge.

Ammolite doublet, showing many color and swirl features. Author's own, available for setting in jewelry.

Fossilized ammonite shell is comprised primarily of aragonite, a type of carbonate mineral. Aragonite typically converts to calcite soon after burial, but in this area it did not. The depth and rate of burial, with contribution of deep hot fluids, prevented the conversion and created ammolite. Stacked thin platelets of aragonite produce the colors. The aragonite forms thin lamellae of various thicknesses. As light enters the thin, regularly spaced layers, diffraction occurs, resulting in flashes of spectral colors. The thicker the layers, the more reds and greens are produced, the thinner the layers, the more blues, and violets. The finer layers responsible for the blues / violets are quite fragile, making them very rare. Individual stones can display the full rainbow or be limited to just one or two colors. The more colors visible, especially if they include blue & purple, the more valuable the piece. Pieces can be either fragments of the original fossil, or an entire fossil specimen.

Paleogeographic reconstruction of the Late Cretaceous (Scotese, 2001). Star indicates location of the mining area.

Along the St. Mary River in SW Alberta, 2 zones of the Bearpaw Formation, contain these gems. About 80 –70 million years ago, as the Rocky Mountains grew, an oceanic waterway was created that ran North-South from the Gulf of Mexico to the Arctic, called the Cretaceous Interior Seaway. Ammonites swam and lived in this seaway. When individuals died, they sank to the bottom and were covered by mud and volcanic ash which became the Bearpaw Fm. As the Rockies evolved, this rock was first buried to a depth of about 4 km, then uplifted and partially exposed.

Ammolite piece showing mostly red, with yellow and green. Author's own, available for setting in jewelry.

Today, the formation dips to the west, diving beneath the mountains, resulting in a limited area between the outcrop and where rock is buried too deep to mine. Ammolite is found in two distinct horizons of the Bearpaw Fm, termed the K Zone and the Blue Zone. The K Zone sits 15-30 meters below the top of the Bearpaw Formation. In it, ammolite is found in siderite concretions and is usually cracked and fractured—this is termed dragon-skin material. The fractures were subsequently “healed” with carbonate and/or clay matrix material. In the Blue Zone, which sits 20 meters below the K Zone, ammolite is found with a thin layer of iron pyrite. The Blue Zone is sometimes termed the sheet zone as the pieces from this zone have with few or no fractures. This zone holds the majority of high grade ammolite Both productive zones in the Bearpaw Formation are only a few meters thick. These special factors are why the gem ammolite is so rare, even though ammonite fossils can be found worldwide. You can find iridescent fossil ammonites, but these are dull compared to actual ammolite gem.

Incredible example of full fossil ammonite covered in ammolite, from Aurora Mine. Available from Aurora.

Much of the productive area sits on a reserve of the Blood Tribe (Káínai), part of the Blackfoot Confederacy. Outside of the reservation, mining rights are owned by two companies - Aurora and Korite. The mined material is screened by hand in both areas. Only about 50% of the mined material is jewelry quality. In the past, ammolite had, in some circles, been called korite, after the mining company that sold it in jewelry pieces. That term is no longer used.

Although the Blood Tribe (Káínai) have known about the stone for centuries, Canadian scientists from the Geological Survey first described the iridescent ammonite shells in 1908. Ammolite was first used in commercial jewelry in the 1960s when gems were mounted in jewelry were exhibited in a small gem show in Nonton, Alberta.

It was subsequently recognized as an organic gemstone in 1981 by the Coloured Stones Commission of the World Jewellery Confederation (CIBJO). In 2004 it was named as the official gemstone of the Province of Alberta. High-grade gems have a range of strong, bright colors. Low-grade stones will show less vibrant colors in a more limited range. Red and green are more common than blue or purple. Some pieces also have patterns like swirls, tooth marks, “spikes”, or “dragon skin” fractures that also increase their appeal. All but the dragon skin features are derived from the original ammonite’s life (and death) history. The dragon skin features are the result of compression caused by the Rocky Mountain orogeny.

Examples of necklaces and earrings created with Ammolite in fine and sterling silver. Artisan Jewelry Creations.

The aragonite platelets are delicate, so ammolite pieces are typically sold as doublets or triplets. Doublets are freeform sections bonded to a shale backing, to strengthen and protect the piece. Triplets have both a backing and a cap of quartz or synthetic spinel, which allows the brilliant colors to show through while protecting it for everyday use. Heat treatment and irradiation are never used.

I use both in my jewelry. I love how silver gives these colorful gems a neutral background which never competes with the brilliant rainbow. Some triplets are also faceted on the cap, giving even more sparkle and return of light.