Shells and pearls often show iridescent color. The cause of this phenomenon has been attributed to diffraction, both diffraction and interference, or interference alone. We used a shell of the mollusk Pinctada margaritifera, which shows very strong iridescent colors, to study how this color is produced in the layers of nacre in shells. From observations with a scanning electronic microscope (SEM), this particular shell exhibits a very fine scale diffraction grating structure. This suggests that the iridescent color is caused by diffraction, which was demonstrated by an experiment using an argon ion laser illuminating the shell to produce a distinct diffraction image. The strength of the iridescent color can be correlated to both the groove density of the diffraction grating formed by the shell, and the surface quality of the grooves themselves. A shell with a high groove density and a smooth groove surface produces a strong iridescent color.
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