Keio University Scientists Decode the Secret Behind Ammolite's Dazzling Colors

For millions of years, the rolling prairies of southern Alberta have concealed a rainbow beneath the rock. Now, a team of scientists from Japan’s Keio University has revealed exactly how that buried brilliance comes to life.

Ammolite — a rare, iridescent gemstone found only in one corner of the world — gets its vivid colors not from pigment, but from structure. In a new study published in Scientific Reports, researchers used advanced electron microscopy to show that ammolite’s shimmering reds, greens and blues are the result of light interacting with layers inside the fossilized shell of an ancient sea creature.

Ammolite is formed from the fossilized shells of ammonites, spiral-shaped marine mollusks that swam in an inland sea covering what is now Alberta about 70 million years ago. As these creatures died and settled on the seafloor, their shells were buried beneath layers of mud that eventually became shale. Over time, the nacre — or “mother-of-pearl” — that once lined their shells transformed into the brilliant gemstone known as ammolite.

Although similar in composition to the nacre found in abalone and nautilus shells, ammolite is far more colorful. The Keio team discovered why: its internal structure is astonishingly precise. The gemstone is built of microscopic plates of the mineral aragonite stacked in perfectly uniform layers, separated by tiny gaps only about four nanometers wide — roughly 25,000 times thinner than a human hair. When light hits these layers, it bounces between them, causing waves of light to interfere with each other and reflect back different colors.

The color depends on the thickness of the layers. Thicker layers produce fiery reds and oranges, while thinner ones create greens, blues and violets. The result is a gemstone that gleams like a living rainbow.

Ammolite is mined almost exclusively from the Bearpaw Formation in southern Alberta, making it one of the world’s rarest gemstones. More than 90 percent of gem-quality material is produced by a single company, Korite International. In recognition of its unique heritage, Alberta officially designated ammolite as its provincial gemstone in 2022.

Beyond jewelry, ammolite holds deep cultural significance. The Blackfoot and other Plains First Nations have collected ammonite fossils for centuries, calling them Iniskim or “buffalo stones,” and using them in ceremonies to ensure prosperity and abundance.

The Keio scientists believe their findings could be used to develop new non-fading color coatings for industrial applications, such as paints.

Credit: Ammolite image (cropped) by A Cynical Idealist, CC BY 4.0, via Wikimedia Commons.