Delete this if you don't want to read about technicalities...
 
John Cramer wrote,
"Andrew Rindsberg's response to Paul Monfils was along the same lines I was
thinking.  I presume the iron would be in small crystals in the top shell
layer.  If so, I should be able to spot them as dark inclusions in the
shell under the SEM.  If iron is distributed at the molecular level, the
layer would have to be viewed optically so the red color could be detected.
Anyone willing to guess which it would be?"
 
Iron substituting for calcium in calcium carbonate would not make the shell
red. It might make it a bit greenish, because this would have to be iron in
the +2 valence state. In rust (hematite), iron has a valence of +3.
 
An SEM would work to spot hematite crystals in situ. X-ray diffraction
would more easily determine the presence or absence of hematite, but
couldn't tell you where the hematite was within the shell (holes, layers,
interstices, etc.). Chemical analysis would be required to tell how much
iron was present.
 
Andrew K. Rindsberg
Geological Survey of Alabama