I have mentioned an interest in windowpane oyster pearls already—here are my reasons:
To begin with, the brightest among my reference lot of 'nacreous' pearls turned out to not be nacre at all, but closest to the shell material typically made by windowpane oysters. Its color—light silver rather than white—comes across fairly well in my latest shot below:
However, it proved impossible to get both the ice-like transparency and the sharp reflection off the surface of this tricky pearl in the same shot. It must be possible—just not today. Size does not help, for sure:
The shell in the picture is a well-weathered Japanese beach find. The pearl has formed off the coast of Bahrain. The two could not have possibly met before getting to be research material. Would it be possible to tell what species the pearl comes from? No, and not for lack of trying…
The fabric pattern of calcite in the pearl ought to be its signature. Indeed, its thin sheets made of roughly rectangular tiles, is typical for all windowpane oysters. Yet, the subtler details that could tell species apart—the thickness of the calcite layers, then the shape, size, orientation and texture of the tiles they are made of—may vary more among natural pearls from the same species than between the shells of different ones. Furthermore, both pearls and shell contain more than one mineral fabric pattern. Sum total, it would take a hefty set of reference samples to get a good idea of the many possible appearances of windowpane pearls, even if precise species identification were not of interest. I rather doubt that my one pearl exemplifies the best.
I can give quite a few reasons for wishing to see more natural pearls of foliate calcite from windowpane oysters. Of course, the one I have will always hold its own, if only on account of the surprise of finding it mid-way through a long run through varieties of nacre—nacre—nacre—nacre… samples of Bahraini P. radiata pearls. Then, shells made of the same type of calcite seem capable of an intriguing range of colors (local Anomia -s)—that I would expect in pearls as well. Besides, slight anomalies of mineralization, easily possible in natural pearls, could yield a miracle or two, perhaps erring on the side of iridescence: barely there in my shell, not apparent on the pearl, equal to the best of all shells in synthetic calcite modelled after this shell material.
As far as I understand, there is yet some chance to see more windowpane pearls—natural and otherwise. There is recent work to followup on:
Rahman, Md Ataur, et al. "Availability of pearl producing marine bivalves in south-eastern coast of Bangladesh and culture potentialities." Journal of Fisheries 3.3 (2015): 293–296.
Report: "From the reared oyster, highest 54 nos. small pearls in the month of April and lowest 7 pearls in December from a single P. placenta were obtained. The study proved that pearls can be obtained from the marine oysters in captivity in Bangladesh, and this offers large scale culture potentialities in our coast."