Iridescent ammonites are fossilised shells of an extinct group of marine animals called ammonites, which lived during the Mesozoic Era, between 240 - 65 million years ago. Ammonites were closely related to modern-day cephalopods such as squid, octopi, and nautili, but they had a distinctive coiled shell that could grow to more than two meters in diameter.

Rare ‘Blue’ Iridescent Ammonite, 42 x 45 cm

What makes iridescent ammonites so fascinating is their remarkable preservation of colourful, shimmering hues on their shell surface. These colours are created by a combination of structural and chemical properties of the shell, which reflect light in unique ways. The rarest and most sought after colour found in these fossils is the deep shape of ultramarine, a colour sought after by fossil collectors across the globe.

Iridescent Ammonite Plaque, 70 x 62 x 4 cm

The iridescence of ammonites is caused by ammolite, which contains a phenomenon called iridescence interference. This occurs when light waves are scattered and refracted by the microscopic structure of the shell material. The shell is made up of layers of aragonite, a mineral that has a unique crystal structure that reflects light. As light waves pass through these layers, they interfere with each other, creating a spectrum of colours that change depending on the angle of the light source and the observer.

The colours found in our specimens are cause by the combination of millions of years of compression and the mineralisation of iron, copper and silica which precipitated from volcanic ash. Ammolite is found primarily in the Rocky Mountains of North America, and it is only mined in a few locations in Canada. The gemstone is highly prized by collectors and jewellry designers for its striking beauty and unique patterns of colour.

The iridescence of ammonites is not only aesthetically pleasing but also provides important information to palaeontologists. The colours and patterns on the shell can help identify different species and provide clues about the environmental conditions in which they lived. Some researchers even use the iridescence of ammonites to study the evolution of colour and optical properties in animals.

Today, iridescent ammonites can be found in museums and private collections around the world, where they continue to captivate and inspire people with their beauty and scientific significance.

Iridescent ammonites continue to do incredibly well at auction. In October 2020, the record sale for this collectible fossil was at Christie’s ‘Sculpted By Nature’ sale, where a large ammonite was sold for £325,000 - far exceeding it’s estimate of £60,000 - £90,000.

Following this, in December of 2022, Sothebys sold a large iridescent ammonite for approx. $201,000 (£162,000), smashing the pre-auction estimate of $120,000 - $180,000.

Our collection of iridescent ammonites and other rare natural history pieces can be viewed in our Green Street Gallery today, open Wednesday to Sunday 12-5 PM.

You can also view our extraordinary collection of fossils and minerals online.

The Wonder of Iridescent Ammonites