PAUL D STEWART / SCIENCE PHOTO LIBRARY PAUL D STEWART / SCIENCE PHOTO LIBRARY
Placenticeras Ammonite shell (upper Cretaceous 75 mya) showing iridescent ammolite gem skin, cracked and flaked in the typical "dragon skin" pattern. Ammolite is a gemstone found primarily in the Canadian Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. The bright colour is because the thickness of the aragonite platelets composing the layer is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours.
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