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Close-up showing the iridescence effect of ammolite from the famous fossil site in Alberta, Canada. Ammolite is a rare kind of gemstone, valued for its vibrant red, green, and blue colors. The most famous specimens come from the curled shells of ammonites, an extinct group related to modern molluscs. The colors are linked to a thin layer of nacre — also known as mother-of-pearl — forming the inner shell layer. Usually this layer decays quickly after the death of an animal and is only rarely preserved in the fossil record. There are only a few places in the world where ammolite can be found, including the St. Mary River in Alberta, Canada, Oklahoma’s Arbuckle Mountains and the island of Madagascar. In living molluscs, the nacreous layer of the shell is composed of thin plate-shaped aragonite crystals embedded in a matrix of organic matter. A beam of white light entering the nacre is reflected and scattered between the aragonite crystals and the organic compounds, resulting in a spectrum of interference colors. However, a spectral analysis of the colors shown by ammolite reveals that they are different from the coloration of other kinds of shells. A Japanese research team lead by Naoki Hizukuri, an engineer at the Faculty of Science and Technology, Keio University, used electron microscopy and computer simulations to investigate the secret behind the brilliant colors. Similar to other shells, the nacre layer in ammonites is formed by plate-shaped aragonite crystals. However, the size of these crystals is very homogeneous, as are the gaps — only 4 nanometers (equal to one billionth of a meter) wide — separating them. The resulting laminated structure is very efficient in reflecting different wavelenghts of light, resulting in a wide and rapidly changing spectrum of colors. The lack of organic material in the fossil shells further enhances color brightness. MORE FOR YOU The full study, "Brilliant structural colors originating from reflection by nanogaps of nacreous layers in fossilized ammonite shells," was published in the journal scientific reports and can be found online here. Editorial StandardsReprints & Permissions
