Ammolite, a trade name for the fossilised shells of two species of ammonite, is an organic gemstone whose iridescent colours dazzle the eyes of gemstone enthusiasts worldwide. The shell of this gem boasts a full spectrum of colours that dance in the light, much like the interior of a vibrant rainbow. Reds, yellows, and greens are the characteristic hues, while blues and violets are the rare treats. This visual spectacle owes its existence to a process known as thin-film interference, an effect caused by the reflection of light from multiple layers of uniform thickness.
This gemstone's two categories provide an even more delightful display of nature's artistry. Fractured ammolite presents a stained-glass window effect, due to the crushed nature of the shell, while sheet ammolite is an unbroken shell of continuous colours. The hard but thin layers of the shell are often left attached to the shale matrix for stability or stabilised with resin and then polished. This process results in gems that are typically less than 3 mm thick after polishing. The gemstone is primarily used in pendants and earrings, providing an added layer of protection. Rarity is the name of the game with ammolite, making it a prized possession among gem collectors.
Besides the trade name ammolite, this gemstone is also known as calcentine and is sold under the trade name Korite. The latter name is derived from the mining company Korite International Ltd., which is the largest commercial supplier of ammolite. In the language of the native Blackfoot people, known as Siksika, the stone is referred to as aapoak, which translates to "small, crawling stone." Ammolite and ammonite are terms that are sometimes used interchangeably, but it's essential to know the difference: ammonite refers to the fossil shells of ammonites, gem-quality or not, while ammolite refers to the gem-quality material made from the fossils of particular species of ammonites.
The colours of ammolite are a visual feast, spanning the full spectrum of the rainbow. Red, yellow, and green are the most common hues found in ammolite, while blue and violet are less common. Their scarcity makes them highly sought after. It is important to note that the colours are naturally occurring and not enhanced.
The only source of ammolite is the Bearpaw Formation, a geological formation that spans from Alberta in Canada to Montana in the USA. However, only the shales in Alberta serve as the mining source for this gemstone. The fossils that produce ammolite are approximately 71 million years old and are found in ironstone nodules.
Ammolite belongs to the aragonite mineral category, which is primarily composed of calcium carbonate.
One of the fascinating aspects of ammolite is that it comes from the fossilised shells of extinct sea creatures, the ammonites. These shells are an astounding 71 million years old, adding a prehistoric allure to this gemstone. Also, the gemstone only became known in the early 1900s and wasn't used as a gem material until 1962, making its rise to popularity quite fast.
Ammolite's value is greatly influenced by its rarity. Only about 5% of the ammonites found in Alberta, Canada, produce any gem-quality material, and from those, only approximately 20% of the shell is usable. These percentages highlight the rarity of this gem, contributing to its high value especially for high quality materials.
The quality of an ammolite gemstone is determined by the brilliance and vibrancy of its iridescence, the number of colours displayed, and the degree of chromatic shift. Chromatic shift refers to the change in colour as the angle of incident light changes, which can be monochromatic (within the same colour group), dichromatic (shifting from one primary colour to another), or spectrochromatic (shifting through the entire spectrum depending on the light source and angle of observation).
To improve the durability of the thin ammolite shell, it is often left attached to the shale matrix or stabilised with a resin. Ammolites are usually assembled as doublets with a shale backing, or triplets capped by a thin colourless layer of quartz or synthetic spinel. This increases the gem's thickness and durability. Some gems also undergo stabilization using Opticon Fracture Sealer & Hardener.
Although synthetic versions of ammolites exist, they are not realistic looking. If the piece has highly uniform pattern and vibrant colours but is selling for a relatively low price, it's likely that it's a fake.
Given its organic nature and relative softness, with a hardness of 3.5-4 on the Mohs scale, ammolite requires careful handling. It should be cleaned with a damp soft cloth, avoiding the use of ultrasonic or steam cleaners. Exposure to heat and acidic substances, such as hairspray and perfume, should be avoided to prevent potential damage.
Ammolite's vibrant iridescence and unique appearance make it stand out among other gemstones. However, its brilliant play of colours can be somewhat similar to opal or labradorite, although these gemstones differ in other key characteristics.
Ammolite is closely related to pearls, due to their shared major constituent mineral, aragonite. However, their appearances, uses, and histories are vastly different.