Nacre
However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such as flame structure as is found in conch pearls.
Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae.
[3] These layers are separated by sheets of organic matrix (interfaces) composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins).
The crystallographic c-axis points approximately perpendicular to the shell wall, but the direction of the other axes varies between groups.
[4][5] In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis that is inclined this way.
[6] This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa and a yield stress of roughly 70 MPa (when dry).
[7] Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation.
This structure, spanning multiple length sizes, greatly increases its toughness, making it almost as strong as silicon.
[12] Tensile, shear, and compression tests, Weibull analysis, nanoindentation, and other techniques have all been used to probe the mechanical properties of nacre.
The adhesive force needed to separate the proteinaceous and the aragonite phases is high, indicating that there are molecular interactions between the components.
[16] Unlike in traditional synthetic composites, the aragonite in nacre forms bridges between individual tablets, so the structure is not only held together by the strong adhesion of the ceramic phase to the organic one, but also by these connecting nanoscale features.
[16][13] As plastic deformation starts, the mineral bridges may break, creating small asperities that roughen the aragonite-protein interface.
To achieve these effects, researchers take inspiration from nacre and use synthetic ceramics and polymers to mimic the "brick-and-mortar" structure, mineral bridges, and other hierarchical features.
[20] But, higher statistical variations generates very weak regions which allows the crack to propagate without much resistance causing the fracture toughness to decrease.
It has been observed in Pinna nobilis, where it starts as tiny particles (~50–80 nm) grouping together inside a natural material.
[25][26] What sets nacre apart from fibrous aragonite, a similarly formed but brittle mineral, is the speed at which it grows in a certain direction (roughly perpendicular to the shell).
Defects that spiralled in opposite directions created distortions in the material that drew them towards each other as the layers built up until they merged and cancelled each other out.
The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre, forming either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues.
[citation needed] At the end of 19th century, Anukul Munsi was the first accomplished artist who successfully carved the shells of oysters to give a shape of human being which led to the invention of new horizon in Indian contemporary art.
Numerous illustrious figures, such as Satyajit Ray, Bidhan Chandra Roy, Barrister Subodh Chandra Roy, Subho Tagore, Humayun Kabir, Jehangir Kabir, as well as his elder brother Annada Munshi, were among the patrons of his works of art.
The biotech company Marine Biomedical, formed by a collaboration between the University of Western Australia Medical School and a Broome pearling business, is as of 2021[update] developing a product nacre to create "PearlBone", which could be used on patients needing bone grafting and reconstructive surgery.
