The shell is constructed in successive layers of calcite, a mineral substance secreted by the glands located in the posterior region of the mollusc. Calcite binds to aragonite fibers to form concholine, which also acts as a support for other mineral particles. The shells are colored with various pigments, and in particular carotenoids (for colors with a yellow base), melanins (for black and brown), porphyrins (for green), indigoids (for blue and red). The iridescences are added to the color, which the light assumes by passing through the layers of aragonite and calcite that form the shell. Sometimes the color may also depend on the animal's diet.
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Some iridescent colors however, for example those of the abalone, do not derive from pigments, but exclusively from the refraction of light. Often these colors change if you change the angle from which you look at the shell, or they can give a rainbow effect. Molluscs use color to blend in, protect their tissues from sunlight, but also to reinforce the structure. However, colors can also appear as a secondary effect, when the animal eliminates toxic substances.
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