For years, ammolite—a gemstone prized for its dazzling rainbow shimmer—has held a geological mystery: what gives it this breathtaking iridescence? Unlike other fossils or seashells with similar mother-of-pearl layers (nacre), ammolite bursts forth in vibrant hues. Scientists finally cracked the code, revealing the precise arrangement of microscopic crystals responsible for this unique phenomenon.
Ammolite originates from fossilized ammonites—ancient squid-like creatures that thrived millions of years ago. These fossils aren’t just pretty; they are packed with layers of aragonite crystal platelets, which make up nacre. But what sets ammolite apart? It wasn’t the mineral itself, but how those crystals were stacked and arranged.
The secret lies in the gaps between these aragonite plates. Researchers from Keio University in Japan meticulously examined ammolite specimens using powerful electron microscopes. They discovered that within ammolite, these gaps are consistently just four nanometers wide— incredibly small! In contrast, similar nacre structures in abalone shells have 11-nanometer gaps, and duller ammonite fossils from Madagascar had collapsed plates, leaving no space at all between the crystals.
These minuscule air pockets play a crucial role in producing color. Think of it like tiny prisms within the fossil: thinner aragonite layers reflect shorter wavelengths of light, creating rich blues. Thicker layers, on the other hand, bounce back longer wavelengths, resulting in deep reds. When these varying colors shimmer together, they produce the dazzling rainbow effect that makes ammolite so captivating.
This discovery was further confirmed through computer simulations. The researchers found that 4-nanometer gaps were the optimal width for creating distinct and vibrant colors. Too narrow or too wide, and the light scattering would be muddled, resulting in a duller appearance. They also noted that uniform thickness across the layers within a single piece of ammolite contributed to its brilliance.
Dr. Hiroaki Imai, lead researcher on the project, believes the specific species of ammonite and fossilization conditions could influence the vibrant color development. His team now plans to apply their findings to another colorful natural phenomenon: opals. These silica gems also exhibit structural colors, and the team hopes to uncover if similar principles govern their dazzling displays.
