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Ana Vasiliu Blog

One more layer

Ana Vasiliu

Last month, I had tried and failed to find nacre in a certain muscle pearl, rather unlike better GIA samples had me hope for. Now, I do have it, with rather nice high resolution shots. In the end, the pearl has interesting news for me.

Pteria sterna 'muscle pearl' – an inner layer exposed for for higher resolution shots. Sample from the IACT research collection; source: Perlas del Mar de Cortez.

Pteria sterna 'muscle pearl' – an inner layer exposed for for higher resolution shots. Sample from the IACT research collection; source: Perlas del Mar de Cortez.

A series of photomicrographs of the small pearls formed under, within, or near the edge of the adductor attachment area of the—by now much sung—Pteria sterna shell, did show glitter under the glass-like cover of transparent fibrous aragonite. Of course, iridescent colors are expected of nacre, if other shell materials are any standard. In 'Pearls of many layers' I reported that nacre was simply not there within some decent depth of this pearl. Of course, inspecting the structure of pearl materials with the SEM ought to be more precise than guessing them by color ! But then again, GIA and Jameson had very good evidence that pearls that are formed within Pinctada maxima and P. margaritifera (close enough!) muscles, ought to contain nacre. A session for higher resolution microscopy was booked, and my oddball sample proved itself: under the glass-like outer layer, there is nacre indeed:

Please notice the transparent layer of prismatic aragonite—rather than reflective, iridescent nacre—buckled in the gap between the pearl and shell. The layer covers the pearl completely, forming a blister pearl. The same pearl appears in frame #B on an overview shot of the shell, here.  

Please notice the transparent layer of prismatic aragonite—rather than reflective, iridescent nacre—buckled in the gap between the pearl and shell. The layer covers the pearl completely, forming a blister pearl. The same pearl appears in frame #B on an overview shot of the shell, here.

 

A high resolution electron micrograph taken on the side of this pearl's layer exposed in the black-and-white shot above.

A high resolution electron micrograph taken on the side of this pearl's layer exposed in the black-and-white shot above.

This nacre is not quite textbook-quality: the defining brick-wall-like pattern of tablets is there, but the tablets are thick and coarse relative to the shell's. Elsewhere on the pearl, tablets appear even more modified. It is by no means common to see such detailed transitions to nacre! There are many more such transition areas throughout this shell, and within its few dozen pearls - all suitable for similar scrutiny.

It is very tempting to expect modified nacre throughout the finer, larger 'muscle pearls' of the world, should any more of these pass through my lab.