Yes, dark-colored shells (not just cowries) can be changed into "golden
forms" or "red forms" by baking for 30 minutes at 300 degrees Farenheit.
Also, a light-colored shell can be changed into an "albinistic" specimen by
the same process.  The reason is that many shell pigments are not heat
stable, and break down into uncolored substances or differently-colored
substances when exposed to substantial heat.  I did a bit of experimenting
with this a few years back, after I read some reference to it.  In a light
colored shell like Conus textile, Cypraea miliaris, or Oliva reticularis, the
pigmentation may be completely destroyed, resulting in a pure white shell.
This can be a problem because, for example, Cypraea eburnea is essentially a
white Cypraea miliaris; and Oliva olorinella is a white Oliva reticularis.
Such white specimens usually have higher commercial value than the
"normal-colored" specimens.  In a dark shell like Conus marmoreus (white
tents on a nearly black ground color), the pigments are not usually
completely destroyed by heating.  Such a shell may come out with a reddish
ground color, the white tents remaining unchanged.  Interesting, since I have
occasionally seen "red form" Conus marmoreus offered, at high prices.  The
idea of looking for fine crackling caused by the heat may be useful for
highly glossy shells like cowries and olives, but not for a murex, volute,
cone, etc.  And, with sufficient care, such cracking can be largely avoided
even in cowries and olives.  I found that two elements were most critical -
changing the temperature gradually, and avoiding direct contact of the shell
with hot metal.  The first consideration was addressed by placing the shell
in a room temperature ove before opening the door.  I addressed the second
issue by placing the shell in a glass (Pyrex, Kimax, or Corning Ware) dish,
which worked quite well, or on an asbestos pad, which worked best.  I really
don't know if anyone has attempted to pass off as genuine shells altered in
this way.  But I am taking a closer look at albinistic and other unusually
colored specimens before I lay out any money for them.  One albinistic shell
may be worth a hundred times as much as a normal colored specimen.  Therefore
a dishonest person could theoretically make many attempts to get a good
looking fake, and still make a tidy profit if he produced a single passable
specimen.
Paul M.