Dear Tom;-
    Just a note to add information. Here in Cincinnati, Our fossils are about
250 to 350million years old: Ordivician. BUT I have a beautiful trilobite that
comes from a clay matrix. Our layers are limestome intersperced with clay
layers. The fossils from the clay are always a lot better than those from the
limestone. The color is consistant---gray.
        Art

Thomas E. Eichhorst wrote:

> Jiongtao,
>
> I am leaving this on the list so others can jump in and point out any errors
> I may make.
>
> I believe the reason the subfossil shells are in such good shape is that
> they are buried.  Once buried the shell is protected from oxidation and
> erosion.  A dead shell that is not buried will quickly have all of the
> things happen to it that "ruins" the shell for collectors.  Worms,
> barnacles, corals, sponges, etc. will build on it and chemically it will
> begin to breakdown (much faster for a gastropod than something like a oyster
> because of the material used to make the shell).  If the shell happens to be
> anywhere near wave action it will also be abraded by wave action in the
> sand.  I saw an experiment once where a cowrie completely lost its gloss in
> less than an hour or so of wave action in beach sand.
>
> Anyway, all of this is avoided or slowed way down if the shell is buried.
> Once the shell is buried, the lack of oxygen stops other critters from
> building on it and slows down the destructive chemical processes that will
> ruin the finish.  However, "slows down" is not stopped, and the shell does
> change.  As you have seen from the shells you got from Don Pisor, the colors
> fade even though the glossy surface remains intact.  That glossy surface is
> a protective coating on the shell and continues to function even after the
> animal is dead.  You can find fossil Cypraea and Oliva from millions of
> years ago that are pretty much grey in color but the glossy surface is still
> there.  In fact, if you are interested I can send you a couple -- I don't
> think I have any extra Cypraea but I am sure I have an Oliva around here.
> Write me off line at [log in to unmask]
>
> Tom Eichhorst in New Mexico, USA
>
> >But I wonder why these "subfossil cowries" have excellent
> > glossy surface and without any worm holes. Most of they have beautiful
> > colorations.