Paul Monfils lists three bleaching processes. A fourth process is
diagenesis, that is, the slow bio/geo/chemical processes that occur when a
shell is buried for thousands or millions of years. If a shell is buried
soon after death in a sediment that seals out the movement of water and
oxygen, then some of its color may be preserved even after hundreds of
millions of years underground. However, most pigments are organic molecules
that eventually break down even when sealed away in this manner. So the
colors in fossil shells may not truly match the original colors, if a
pigment has altered to another color, or if only one of several original
pigments is preserved, or if a new pigment infiltrates the shell from
outside. Shells, modern and fossil, are often stained black or red with
iron compounds, as we discussed in another Conch-L thread a few months ago.
Some shells contain brown iron hydroxides (e.g., some species of brachiopod
Lingula), and these inorganic molecules are relatively stable. It is not
very unusual to find brown-banded lingulids in Cambrian rocks, half a
billion years old. But whether the lingulids looked just like this when
they were alive, no one can say.
 
Except for iron hydroxide, the colors in fossil shells are very susceptible
to fading after they have been exposed to light. If you have such a
specimen, keep it in a dark place rather than putting it on permanent
display. Once the colors have faded, they may still be discerned under
ultraviolet light, which makes some organic molecules fluoresce.
 
Andrew K. Rindsberg
Geological Survey of Alabama