Ammolite is a rare, iridescent organic gemstone, highly prized for its vibrant display of color. Unlike most gems, ammolite’s beauty originates from a biological process rather than purely geological mineral formation.
Geological Origins and Formation
Ammolite originates from the fossilized shells of extinct marine mollusks called ammonites, specifically species like Placenticeras meeki and Placenticeras intercalare. These cephalopods thrived in a prehistoric inland subtropical sea that once covered parts of North America, known as the Western Interior Seaway. When ammonites died, their shells sank to the seabed and were subsequently buried by layers of bentonitic mud, which eventually transformed into shale.
Over millions of years, specific geological conditions led to the preservation of the shell’s aragonite, the primary mineral in ammolite. The combination of pressure, mineralization processes, and the surrounding sediments prevented the aragonite from converting into calcite. Significant deposits of gem-quality ammolite are found almost exclusively within the Bearpaw Formation, located along the eastern slopes of the Rocky Mountains in Alberta, Canada. Only a small fraction of the ammonite fossils found in this region yield the gem-quality material.
Distinctive Characteristics
The most striking feature of ammolite is its vibrant, iridescent play of color, a phenomenon known as labradorescence or interference. This visual effect is not due to light absorption, but rather to the interaction of light with the microscopic layers of aragonite platelets within the fossilized shell. As light passes through these incredibly thin, stacked layers, it diffracts and interferes, producing a spectrum of hues that shift with viewing angle and light source.
Ammolite can display all spectral colors, though red and green are more common, while blue and purple appear less frequently. The thickness of the aragonite layers influences the colors observed; thicker layers tend to produce reds and greens, while thinner layers yield blues and violets. Ammolite often exhibits distinct patterns within its iridescence, such as “dragon skin,” which results from natural fractures, or “stained glass” effects. The gem’s composition is primarily aragonite, along with trace amounts of other minerals like calcite, silica, and pyrite, and elements such as aluminum, iron, and magnesium.
Applications and Value
Ammolite is primarily used in jewelry, fashioned into pieces like pendants, earrings, brooches, and rings. Beyond adornment, larger or particularly well-preserved ammolite specimens are also sought after as decorative objects or for mineral collections.
Several factors determine ammolite’s value.
- The intensity and brightness of its colors, with highly vivid hues commanding higher prices.
- The breadth of the color spectrum, particularly multiple primary colors or rare shades like crimson, violet, or gold.
- Chromatic shift (how colors change with viewing angle) and rotational range (how much the stone can be turned while maintaining its color display).
- The size of the gemstone, absence of cracks or inclusions, and whether the ammolite layer is thick enough to stand alone without extensive backing.
While there isn’t a universally standardized grading system, common classifications include grades like AA, A+, A, and A-.
Care and Preservation
Ammolite is a relatively delicate gemstone, possessing a Mohs hardness ranging from 3.5 to 4.5. This softness makes it susceptible to damage from impacts, scratches, harsh chemicals, and extreme temperature fluctuations. Cleaning ammolite should be done gently, using a soft cloth and mild soap with lukewarm water.
It is important to avoid ultrasonic cleaners, steam cleaners, abrasive materials, and exposure to chemicals such as hairsprays, perfumes, and acids, as these can dull or damage the gemstone. When not being worn, ammolite jewelry should be stored separately in a soft pouch or jewelry box. For pieces like rings, which are more prone to impact, ammolite is often set with a protective cap or incorporated into a doublet or triplet construction.