Dear Conch-L readers,

I am writing my disseration on shell bead production in prehistoric Ecuador.
I am hoping that you can help me fill in the gaps in knowledge of shell
structure.

One of the main subjects of the research is beads made from Spondylus sp. In
Ecuador the species involved are Spondylus princeps and Spondylus calcifer
(and possibly Spondylus leucacanthus). Many of the beads that I have
recovered from archaeological sites (most dating to less than 2000 years
ago) are what I call ROP beads. ROP beads are red, orange, purple or a
combination of the three colors since often a single bead will have more
than one color. These colors occur mainly in the outer crystalline layer of
the shell as well as along the margin.

From my inexperienced view, the ROP beads have a texture that white beads
(which could have either been made from the interior of Spondylus shells of
from other species) do not have. I know that shells can be made from
aragonite and calcite (and sometimes vaterite) and that there are three
basic types of structures, including prismatic, foliate or laminar and
crossed lamelar. However, on an article dedicated to Spondylus, Skoglund and
Mulliner indicate that Spondylus lacks "simple, prsimatic calcite;
crossed-lamellar aragonite prominent". The reference is to Waller 1978,
which has a single paragraph description that includes this statement. This
makes it sound like the strucure of the Spondylus shell is relatively
uniform. If this is true, then there should be no difference in the
crystalline structure of the inner and outer crystalline layers. This would
seem to indicate that my observation that the colored part of the Spondylus
shell has a different texture and, therefore a different crystalline
structure, is incorrect.

I am attempting to show why these ROP beads are much more apt to break
during production and, based upon my own qualitative observations, I believe
this has to do with the 'texture' of the crystallin structure of the bead.
Can anyone enlighten me as to why the texture seems so different when (based
upon on citation) the crystalline structure is supposed to be the same.

Thanks for any input.

Ben Carter

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