Ammolite is a distinct organic gemstone, celebrated for its vibrant, iridescent colors that shift and shimmer with the light. Unlike most gemstones formed from minerals deep within the Earth, ammolite originates from the fossilized shells of ancient marine creatures. Its striking visual appeal and unique biological heritage set it apart.
The Exclusive Source
The primary commercial source of gem-quality ammolite is almost exclusively found in a specific region of Alberta, Canada. Mining operations are concentrated along the eastern slopes of the Rocky Mountains, particularly near the St. Mary River, south of Lethbridge. While small deposits exist elsewhere, such as Montana, England, and Madagascar, these typically yield material too fragile or lacking the intense iridescence found in Canadian ammolite.
Geological Conditions for Formation
Ammolite’s formation is tied to specific geological conditions present in the Bearpaw Formation, a sedimentary rock unit dating back to the Late Cretaceous period, approximately 70 to 75 million years ago. During this time, an ancient inland subtropical sea, known as the Western Interior Seaway, covered much of what is now Alberta. When ammonites, extinct marine mollusks with coiled shells, died, their shells sank to the seafloor.
These shells were then buried rapidly under layers of fine-grained sediment, including bentonitic mud derived from volcanic ash. This rapid burial and the specific chemical environment, characterized by high concentrations of iron and magnesium, helped preserve the original aragonite of the ammonite shells. Aragonite is a form of calcium carbonate that makes up the nacreous layers of the shell, similar to mother-of-pearl.
Over millions of years, the immense pressure and heat from the overlying sediments caused a slight diagenetic alteration of the aragonite. The thin, stacked layers of aragonite platelets interact with light, causing interference and diffraction, which results in the vibrant, shifting colors seen in ammolite.
Global Uniqueness
While ammonite fossils are found globally, the specific geological conditions required for gem-quality ammolite are incredibly rare and almost exclusively present in southern Alberta. The unique combination of rapid burial, specific mineral-rich waters, and subsequent pressure within the Bearpaw Formation allowed the aragonite to persist and develop its optical properties, rather than converting to calcite or pyrite, which commonly happens in other fossilization processes. This distinct preservation process means that only an estimated 5% of ammonite fossils from this region are suitable for gemstone production. The limited geographical extent of these conditions makes ammolite one of the rarest gemstones in the world.