Sea Shell Colours ~ The inner nacre of a sea shell shines with iridescent colours. Imaged by Barbara Grudzinska (flickr).

©Barbara Grudzinska, shown with permission.
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Sea shell nacre is a miracle of bio-engineering. It is made up of polygonal (usually hexagonal) tiles of a very hard crystalline form of calcium carbonate, aragonite, cemented in a thin matrix of conchiolin – polysaccharide and protein fibres. The tiles are each a single crystal and are tessellated in stacked layers. The individual tiles are some 10 micron across.

The structure regularity is remarkable. Protein secretions control its formation. Initially there is random nucleation of aragonite on a thin protein layer. The fastest crystal growth would normally be a direction to thicken the crystals but this is rather neatly prevented by the selective absorption of proteins onto the top crystal faces. Horizontal growth across the layer proceeds instead and contact with adjacent tiles eventually forces a regular spacing. Periodic injections of an inhibitor protein is hypothesised to limit the tile thickness to 0.5 micron. An upper terrace of crystal starts to form on the face of each tile and eventually grows into a second layer. And so on...

The iridescent colours result from multilayer interference and from diffraction by the outer stepped surface of the nacre.  The latter was proved in the 19th Century by David Brewster who made a wax cast of a shell surface. The wax cast also showed iridescent colours!

Layers of aragonite tiles .      Regularly spaced surface grooves
Images ©T L Tan, D Wong & P Lee, Iridescence of a shell of mollusk Haliotis Glabra, Optics Express, Vol. 12, Issue 20, pp. 4847-4854 (2004)