Andrew K. Rindsberg wrote:
>
> 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
 
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