Ammolite is a unique, opal-like organic gemstone prized for its brilliant, iridescent coloration. As a biogenic gem, it is derived from a living organism, similar to pearl or amber. Its spectacular appearance displays a full spectrum of colors that seem to shift when viewed from different angles.
Ammolite is recognized as one of the rarest gemstones, with virtually all commercial-grade material sourced exclusively from a single area in North America. The gem received official gemstone status from the World Jewellery Confederation (CIBJO) in 1981.
Geological Origin and Formation
Ammolite originates from the fossilized shells of extinct marine mollusks called ammonites, which thrived during the Cretaceous period approximately 71 million years ago. The gem material is derived specifically from two species: Placenticeras meeki and Placenticeras intercalare. These cephalopods lived in the Western Interior Seaway, an ancient body of water covering much of central North America.
The only locality producing gem-quality ammolite is the Bearpaw Formation, found along the St. Mary River in southern Alberta, Canada. When these ammonites died, their shells sank and were rapidly buried by layers of fine-grained sediment, volcanic ash, and clay-rich mud, which is now known as the Bearpaw Shale. This quick burial helped preserve the original shell structure.
A combination of high pressure, tectonic activity, and a mineral-rich environment was necessary to transform the shell’s nacre into ammolite. The anoxic, or oxygen-poor, conditions within the sediment prevented the aragonite, the primary mineral in the shell’s nacre, from converting to the more stable mineral calcite. Instead, the aragonite was preserved and mineralized with trace elements like iron and magnesium, fossilizing the delicate nanostructure of the shell.
Distinctive Optical Properties
The striking flash of color exhibited by ammolite is not due to chemical pigments but is a phenomenon known as structural color. This iridescence, sometimes referred to as the “schiller effect” or “fire,” is caused by the interference and diffraction of light. The original nacreous layer of the ammonite shell consists of microscopic platelets of aragonite stacked in ultra-thin, parallel layers.
When white light enters the ammolite, it reflects off these multiple, precisely arranged layers. The interaction of these reflected light waves causes certain wavelengths to be amplified while others are canceled out, creating the intense spectral colors. The specific color observed is directly related to the thickness of these individual aragonite layers.
Thicker layers produce the more common colors, such as red and green, while thinner layers yield the rarer hues, including blue, violet, and crimson. This structural mechanism causes a phenomenon called spectrochromatic shift. As the viewing angle is changed, the apparent color shifts through the visible spectrum, sometimes displaying three-dimensional color changes.
Use in Jewelry and Grading Standards
Ammolite is a delicate material, possessing a low hardness of 3.5 to 4.5 on the Mohs scale, which makes it susceptible to scratching and flaking. Because the gem-quality layer is often less than one millimeter thick, it requires stabilization before it can be used in jewelry. The most common stabilization methods involve creating assembled stones called doublets or triplets.
A doublet is made by bonding the thin ammolite layer to a stronger backing, often using the natural shale matrix, black onyx, or glass. A triplet adds a transparent cap, typically made of clear quartz or synthetic spinel, over the ammolite layer for maximum protection. This capping significantly increases the stone’s durability, making it suitable for rings and bracelets.
The commercial value of ammolite is determined by a grading system that assesses several optical factors, though a single universal standard is not used across the entire industry. Major producers use proprietary scales, such as the Korite system, with grades like AAA, AA, A, and Standard. Top-grade ammolite, such as AAA, must display at least three vivid colors, including a rare color like blue or violet.
Other factors influencing the grade include the intensity and brightness of the colors, the continuity of the pattern, and the degree of rotational color shift. Only a small percentage of the material mined qualifies for the highest commercial grades.