Ammolite is a unique and captivating organic gemstone, officially recognized in 1981, that commands attention for its intense, iridescent play of color. This rare material is the fossilized shell of extinct marine mollusks called ammonites, which thrived in ancient seas millions of years ago. Unlike most gemstones formed by purely geological processes, Ammolite is biogenic, meaning it originated from a living organism, placing it in the same category as pearl and amber. Its distinctive rainbow sheen, which can rival that of fine opal, is what makes Ammolite highly prized and sets it apart from non-gem-quality ammonite fossils found worldwide.
The Exclusive Source Region
The world’s commercial supply of gem-quality Ammolite originates almost exclusively from a tiny geographic area in North America. This specific location is the Bearpaw Formation, situated along the St. Mary River in Southern Alberta, Canada. This limited areal extent is the primary reason for the material’s scarcity.
This region was once part of the Western Interior Seaway, a vast, prehistoric inland sea that existed during the Late Cretaceous period. While the Bearpaw Formation stretches across parts of Alberta, Saskatchewan, and Montana, only the deposits in Southern Alberta consistently yield the quality and quantity required for commercial gem production. The Canadian government officially recognizes this localized area as the sole source.
Geological Requirements for Formation
The formation of Ammolite required a rare convergence of biological and geological factors approximately 70 to 75 million years ago. The process began with the death of specific Late Cretaceous ammonites, primarily Placenticeras meeki and Placenticeras intercalare, whose shells sank to the bottom of the ancient Bearpaw Sea.
Rapid burial in fine-grained marine sediment, which eventually became the Bearpaw Shale, was essential for preservation. This quick covering prevented the aragonite shell material from dissolving or converting into the more common, non-iridescent mineral calcite. The shells were subsequently preserved within siderite concretions, which formed protective mineral casings around the fossils.
Over millions of years, the immense weight of overlying rock layers and tectonic pressure from the rising Rocky Mountains heated and compressed the shell layers. This process chemically altered the aragonite, allowing its microstructure to diffract light and produce the full spectrum of vibrant colors seen in Ammolite. The resulting gem material is typically a thin layer, often less than a millimeter thick, attached to its shale or siderite backing.
From Rock to Gem: Mining and Recovery
The recovery of Ammolite is challenging due to the geological layers in which it is found. Commercial mining operations are primarily open-pit extractions conducted along the exposed river valleys and slopes of the Bearpaw Formation. Miners focus on specific layers, such as the “K Zone” and the deeper “Blue Zone,” which contain the highest concentrations of ammonite fossils.
Extraction involves heavy machinery to remove the overburden, but the actual recovery often requires specialized techniques and hand labor. Once the shale layer containing the fossils is exposed, workers must carefully screen and examine the excavated material. This delicate process is necessary because much of the Ammolite is found as fractured fragments, often referred to as “dragon skin” due to its tessellated appearance.
After extraction, the Ammolite material must be stabilized immediately to prevent cracking or delaminating. Due to its relative softness, the thin layer of iridescent aragonite is typically backed with a sturdier material, like shale or resin. It is often capped with a clear protective layer, such as quartz, before it can be cut and polished for use in jewelry.