Nacre

Pinctada margaritifera Shell. Image Courtesy of parels-ael.nl

Pinctada margaritifera Shell.
Image Courtesy of parels-ael.nl

Nacre is the iridescent substance formed from the crystallization of calcium carbonate as aragonite, bound by conchiolin, which composes the outer layer of a pearl (or the lining of a shell.) Nacre quality is an important constituent of pearls.