Pearl
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A pearl is a hard, glistening object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is composed of calcium carbonate in minute crystalline form,
Georgian seed pearl gold ring
A black pearl and a shell of the black-lipped pearl oyster. The iridescent colors originate from nacre layers.
Electron microscopy image of a fractured surface of nacre
Nacre
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Nacre, also known as mother-of-pearl, is an organic–inorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong,
Pearl
…They have also been crushed and used in cosmetics, medicines, and paint formulations. Whether wild or cultured, gem-quality pearls are almost always nacreous and iridescent, like the interior of the shell that produces them. However, almost all species of shelled mollusks are capable of producing pearls…
The iridescent nacre inside a nautilus shell
Nacreous shell worked into a decorative object
Fossil nautiloid shell with original iridescent nacre in fossiliferous asphaltic limestone, Oklahoma. Dated to the late Middle Pennsylvanian, which makes it by far the oldest deposit in the world with aragonitic nacreous shelly fossils.