Gem-quality Ammolite produces a spectacular display of iridescent color when it is observed in reflected light. The colors of an individual stone can run the full range of the visible spectrum or be limited to just one or two colors. The color display can rival fine opal and labradorite in its intensity and beauty.
Ammolite is a trade name given to a thin iridescent aragonite shell material that is found on two species of extinct ammonite fossils (Placenticeras meeki and Placenticeras intercalare). Other less-frequently used trade names for Ammolite are “Calcenite” and “Korite.” It is also known simply as “ammonite shell.”
Ammolite is a rare material. All of the world’s commercial production comes from a small area along the St. Mary River in southwestern Alberta, Canada. There, two companies mine Ammolite from thin layers in the Bearpaw Formation where the ammonite fossils are found.
The story of Ammolite begins about 70 to 75 million years ago when forces within the Earth were building the Rocky Mountains in what is now northwestern North America. The area on the east side of the mountains was covered by a broad body of water known as the Western Interior Seaway. It connected what is now the Gulf of Mexico to the Arctic Ocean.
Rains falling on the eastern flank of the young Rocky Mountains washed sediments into the seaway. These sediments accumulated and eventually formed a rock unit known as the Bearpaw Formation. The Bearpaw is composed primarily of marine shale but contains some thin sandstones and layers of volcanic ash.
The Western Interior Seaway contained many forms of life, including bony fish, shellfish, sharks, sea turtles, and ammonites. Ammonites (the source organisms for Ammolite) are an extinct group of marine invertebrates that possessed a tightly coiled shell similar to the modern Nautilus. The ammonites in the seaway grew to a diameter of up to one meter, but most were about 1/4 to 1/2 that size.
When the ammonites died, their shells fell to the bottom of the seaway and were covered with sediment. Many of them served as a nucleus for the formation of siderite concretions which are now found in the Bearpaw Formation. The ammonite fossils within these concretions sometimes have an outer shell layer composed of gem-quality Ammolite.