Opalized ammonite fossils are some of the most unique and spectacular paleontological specimens. They were formed through a geological process known as opalization, wherein organic material from ancient ammonite shells is replaced by silica, resulting in visually stunning fossils characterized by vibrant colors and intricate patterns.
What are ammonites — and why these fossils are so important
Ammonites are extinct marine mollusks that thrived in Earth’s oceans for over 300 million years before their extinction around 66 million years ago.
They were spiral-shelled creatures, closely related to modern cephalopods like squid and octopuses. In fact, they emerged over 100 million years before octopuses. They had intricate, coiled shells, which often displayed complex suture patterns, and were highly adaptable, inhabiting various marine environments from shallow seas to deep waters.
They’re also one of the most commonly preserved fossils, which is important for geologists. Their fossils give clues regarding their ancient environment and can even help researchers date geological layers. Because some ammonites only lived during specific periods, when geologists find such fossils, they know the age of the geological structure — this makes ammonites index fossils.
Ammonites are primarily found in marine sedimentary deposits, where they were once abundant during the Late Cretaceous period. This was the time of T-Rex when massive dinosaurs roamed the land. Meanwhile, ammonites thrived in the sea. Their fossils can often be located in various geological formations, ranging from deep-sea sediments to shallow deposits.
For ammonite fossils to achieve such exceptional preservation, specific environmental conditions must be met. Rapid burial by sediment is key, as it protects remains from decay and physical disturbances. The conditions of low oxygen levels, or anoxia, in sediment-rich environments also play a significant role, as they help slow down the decomposition processes
While several types of ammonite fossils exist, none are probably more spectacular than opalized ones.
How opalized ammonites are formed
Opal is a hydrated amorphous (non-crystalline) type of silica. The process of opalization refers to something turning into an opal — like for example, a big part of this fossil.
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Opalized ammonite fossils are formed through a unique geological process involving the replacement of organic material with silica, which is the primary component of opal. This process can occur in various environments, but it’s not always the same. It can be influenced by factors such as the original composition of the ammonite, the surrounding sediment, and the availability of silica-rich fluids.
The opalization process occurs through two main mechanisms: internal molding and the complete replacement of organic material. Internal molding preserves the external shape of the ammonite while leaving a cavity filled with silica. Meanwhile, in replacement, the opal is directly infiltrated into the organic material, maintaining fine structural details.
Some, but not all opalized ammonites are iridescent. This happens due to the unique optical properties of opal, which is formed when silica-rich water deposits microscopic silica spheres within the fossil. These spheres diffract light, creating the mesmerizing play of colors. This can result in a brilliant display of shifting hues, making opalized ammonites some of the most visually stunning fossils. However, not all opalized ammonites exhibit iridescence; the effect depends on the specific conditions of opal formation.
Science used as jewelry
It’s not surprising that opalized ammonite fossils, with their striking beauty and unique color patterns, are often cut and polished to create exquisite jewelry. You can find these fossils fashioned into everything from pendants and earrings to rings, and brooches.
In fact, iridiscent opalized ammonites are so striking that they have their very own mineral name: ammolite. Ammolite is biogenic gem material primarily found in Alberta, Canada.
However, these are not particularly durable fossils (or gems). They’ll also be likely cut into thin pieces (to make more of them), so they’re not ideal for everyday wear.
But there’s another reason why you should be wary of these fossils: ethics.
As demand for these unique specimens increases, the potential for over-extraction raises questions about the sustainability of fossil resources and the preservation of important geological sites. Efforts to balance conservation with commercial interests are ongoing, emphasizing the need for responsible practices within the fossil trade.
Our general stance is that special fossils should be kept in museums or other educational facilities, not in personal collections.
Fossils like opalized ammonites offer a glimpse into Earth’s deep past, showcasing the interplay of biology, geology, and chemistry in creating natural marvels. Their scientific value extends far beyond their aesthetic appeal, providing insights into ancient marine ecosystems and the geological processes that preserved them. Whether admired in a museum or studied in a laboratory, these extraordinary specimens remind us of the importance of preserving our natural heritage.
