There exists a much deeper and broader literature base on molluscan 
pigments and color patterns than this single article references. Most 
color and color pattern in molluscan shells is caused by organic 
pigments, the production of which is genetically controlled. The 
yellows, oranges and reds are mostly due to compounds known as polyenes 
but can also be due to the presence of other organic compounds as well. 
These compounds can display many properties including fluorescence but 
fluorescence can also be caused by other factors such as the denaturing 
of proteins like those incorporated into the organic framework of the 
shell biomineral stucture. Physical optical properties can and do play a 
role in coloration as well, but the primary reason for the colors we see 
in the shells is due to pigments. A quick google search of "wet rocks 
more colorful" will take the interested reader to several sites that 
explain why wet shells are more vividly colored, a purely physical 
phenomenon. As Harry just mentioned, "optical grating" phenomenon are 
not a significant source of color in shells but certainly some color 
characteristics are due to optical properties (think mother of pearl). 
Harry, I would propose that the some of the golden yellow color under UV 
that you observe may actually be due to the fluorescence of the mineral 
material that replaced the original biomineral in your samples. In those 
cases, the fluorescence is caused mostly by lattice defects or elemental 
inclusion in the mineral crystal structure. It is known that different 
structures within shells can be composed of varying combinations of 
calcium carbonate polymorphs, perhaps leading to differing dissolution 
and replacement mechanisms.

----------------------------------------------------------------------
[log in to unmask] - a forum for informal discussions on molluscs
To leave this list, click on the following web link:
http://listserv.uga.edu/cgi-bin/wa?SUBED1=conch-l&A=1
Type your email address and name in the appropriate box and
click leave the list.
----------------------------------------------------------------------