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?