Why Ammolite Glows: New Research Explains the Science Behind Its Rainbow Colours

A 2025 study published in Scientific Reports has given collectors a clearer scientific explanation for one of Ammolite’s greatest mysteries: why some fossilized ammonite shell displays such vivid red, green, and blue colours. The paper, titled “Brilliant structural colours originating from reflection by nanogaps of nacreous layers in fossilized ammonite shells,” was published on October 30, 2025 by Naoki Hizukuri and co-authors. 

The key discovery is that Ammolite’s color is structural, not pigment-based. In simple terms, the gemstone does not appear red, green, or blue because it contains red, green, or blue dye-like material. Instead, its colour comes from the way light interacts with extremely thin, regularly stacked layers inside the fossil shell. The researchers showed that Ammolite’s vivid colors are produced by preserved nacre — the same general “mother-of-pearl” type layer found in shells — but fossilized in a special way. 

Ammolite is made from the fossilized nacre of extinct ammonites, especially Placenticeras species from Alberta, Canada. The 2025 study examined brilliantly colored Ammolite from Alberta and compared it with modern shells, such as abalone and nautilus, as well as an iridescent ammonite fossil from Madagascar. The researchers wanted to understand why Ammolite can look much more saturated and jewel-like than ordinary shell nacre. 

Under powerful microscopes, the scientists found that Ammolite still preserves a layered structure of aragonite plates, a crystalline form of calcium carbonate. These plates are stacked like pages in a book. Between the plates are incredibly tiny spaces, called nanogaps, measuring about 4 nanometers wide. A nanometer is one-billionth of a meter, so these gaps are far too small to see with the human eye. 

When light enters this stack of aragonite plates and nanogaps, certain wavelengths of light reflect back more strongly than others. That is what creates Ammolite’s red, green, and blue colours. The study measured strong reflection peaks at about 640 nanometers for red, 540 nanometers for green, and 460 nanometers for blue. These narrow reflection bands help explain why high-quality Ammolite can look so vivid rather than merely pearly or pastel. 

The research also helps explain why different Ammolite colors appear in different pieces, or even in different areas of the same piece. The colour depends partly on the thickness of the aragonite plates. In the samples studied, blue Ammolite had plates averaging about 140 nanometers thick, green about 160 nanometers, and red about 190 nanometers. In other words, tiny changes in the fossil’s internal layering can shift the color seen by the collector. 

One of the most important findings for collectors is that vivid Ammolite requires not only the right layer thickness, but also a very regular and even structure. The researchers found that brilliant colour is associated with a consistent lamination pattern through the nacre. A high-quality green sample showed a uniform structure over more than 400 micrometres, while less vivid material changed more quickly with depth. The paper also notes that polishing can sometimes change the apparent colour, because cutting deeper into the layer may expose a different internal structure. 

This explains something many Ammolite cutters and collectors already appreciate from experience: Ammolite is a gemstone of extraordinary subtlety. Its beauty is not just on the surface. It is controlled by microscopic architecture preserved from an ancient marine animal that lived tens of millions of years ago. Even a small difference in polishing depth can reveal a different colour or alter the strength of the colour display. 

The study also compared Ammolite with abalone and nautilus shells. These modern shells have nacre too, but their colours are usually softer and more pearly. The researchers found that Ammolite’s special brightness comes from a combination of very small nanogaps and a highly consistent layered structure. In abalone, organic material remains between the aragonite plates, and the reflection is weaker. In some other fossil ammonites, the layers may be collapsed or less regular, producing a weaker colour. 

The authors even tested their idea by modifying abalone shell. They removed some organic material and pressed the shell under high pressure. This improved the colour somewhat, but it did not fully recreate Ammolite’s brilliance. That experiment supported the conclusion that Ammolite’s exceptional color depends on a rare combination of fossil preservation, nanoscale gaps, and regular layering. 

For collectors, the main lesson is simple: Ammolite’s colour is the fossil itself interacting with light. The finest pieces are not merely colourful; they are examples of natural optical engineering. Their brilliance comes from fossilized shell layers arranged with nanometer-scale precision, preserved from ancient ammonites and revealed through skilled cutting and polishing.

Collector takeaways

Ammolite colour is structural, not pigment-based. Grinding the material into powder destroyed the visible red, green, or blue effect; the resulting powder appeared pale brown regardless of the original colour. That supports the idea that the colour depends on the structure, not on coloured substances inside the stone. 

Red, green, and blue relate to layer thickness. Slightly thicker aragonite plates tend to reflect longer wavelengths, moving the colour toward red. Slightly thinner plates tend to reflect shorter wavelengths, moving the color toward blue. 

Vivid colour depends on regularity. The more consistent the microscopic layering, the cleaner and stronger the color can appear. Irregular layering can broaden the reflection and create a paler or less saturated appearance. 

Polishing matters. Because the structure can vary with depth, polishing may reveal a different colour layer. This helps explain why cutting Ammolite requires experience and why a single fossil can show multiple colors. 

Ammolite is scientifically rare. Ammonite fossils occur in many parts of the world, but brilliantly coloured Ammolite-like material is known only from certain localities and preservation conditions. The paper specifically highlights the Bearpaw Formation in Alberta as the source of the famous gem material. 

Source Credit

Hizukuri, N., Oshima, Y., Yagi, Y. et al. “Brilliant structural colors originating from reflection by nanogaps of nacreous layers in fossilized ammonite shells.” Scientific Reports 15, Article 37541, 2025. DOI: 10.1038/s41598-025-21872-z