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Ana Vasiliu Blog

Name the Origin of Pearls

Ana Vasiliu

What’s in a name!? Mexican natural pearls (Pteria sterna) are brought to bear on the origin of pearls…. Take a look at what these pearls enclose and coin new language.

Name the Origin of Pearls

Pearls with extraneous things inside had been the model for a century's worth of failed experiments toward pearl culture, ingloriously ended by the invention of grafting—buttressed by rather different ideas. Yet the spectacularly effective if unseen origin of pearls—the natural counterpart of grafting that worked—hasn’t made much of a name for itself. To begin with, it needs a name.

Jameson called it an 'agency'—justified, but vague. Elsewhere, I am using 'natural pearl nuclei'—an unpleasantly ambiguous term hinting at a tangible blob rather than the more likely identity of the origin of pearls somewhere in the context of cell specialization…. I would mind less schlepping around the pompous 'origin of pearls', than technical jargon. You name it. What's in a name…?

Mexican Pteria sterna natural pearls from the research collection at IACT. Source: Perlas del Mar de Cortez. ■ The group of three white shards on top were a round white pearl—an outer layer of nacre has fractured away, exposing the surface of an inner layer: as often the case, their break happened along a major change in the structure of nacre. It took one more break to find out how this pearl started (below). ■ To the left: three of eighty-something small golden pearls found in a single shell—see the first growth of two of them below. ■ The bright oval and the two pieces below were a single pearl; another from the same small lot of 'hinge' pearls broke across—laf is shown with its large, hollow interior facing up. Note that both these pearls enclose empty chambers. ■ The brown pearl looks more iridescent, darker and not necessarily brown in person—the camera would not play along. The large mineral body turned out to be a beautiful surprise, impossible to guess from its rough skin visible here.

Mexican Pteria sterna natural pearls from the research collection at IACT. Source: Perlas del Mar de Cortez. ■ The group of three white shards on top were a round white pearl—an outer layer of nacre has fractured away, exposing the surface of an inner layer: as often the case, their break happened along a major change in the structure of nacre. It took one more break to find out how this pearl started (below). ■ To the left: three of eighty-something small golden pearls found in a single shell—see the first growth of two of them below. ■ The bright oval and the two pieces below were a single pearl; another from the same small lot of 'hinge' pearls broke across—laf is shown with its large, hollow interior facing up. Note that both these pearls enclose empty chambers. ■ The brown pearl looks more iridescent, darker and not necessarily brown in person—the camera would not play along. The large mineral body turned out to be a beautiful surprise, impossible to guess from its rough skin visible here.

Truth be told, the folk who had tried to improve natural pearl harvests by way of finding and spreading the right kind of pearl-inducing parasites did have a point. I am still finding it surprising that this sort of exercise does fail—that the many obvious life events seen to be happening alongside natural pearls, do not induce more pearls if systematically applied: anything works, nothing works predictably… close to coincidence, if not quite down to lottery odds. No, the easiest field reports to build on—rather what you’d expect around a great leap of faith toward new ideas, as the investigation of pearl sacks rather than the pearls themselves was, then, the rather extraordinary leap of faith of grafting. I can testify that it is incomparably easier to break pearls than deal with the pearl sacks—the latter are somewhere in the pipeline.

Intruders in pearls are still giving me a rare marker of some event—the invasion—cleanly preceding the beginning of pearl growth. These hinge pearls are my first and only naturals with foreign objects inside: lace-like diatom skeletons, embedded in a rough, fibrous first layer with or without traces of mineralization.

Below, detail of the dark deposit lining the narrow cavity in the long oval hinge pearl:

Pearl

The large hollow of the small round pearl nearby is lined with somewhat similar material: the rough organic stuff is there, so are diatoms. The white patches of mineralization obvious among the earliest organic deposit in the first pearl do not appear in the second—variety is the rule among natural pearls. Interestingly, the row of long mineral grains recalls stunted prisms (on my To Do list to confirm).

Perhaps no coincidence that the pearl in which mineralization started later comes with a larger hollow—details to keep in mind toward interpreting the large scale features of early pearl growth referenced in pearl gemology.

Pearl

Interestingly, a similar rough, fibrous layer is described at the beginning of a grafted pearl (1, 2).

Similar mineral formations appear, this time with no diatoms or rough fibers associated, as the earliest deposit in the small golden pearls:

The round nucleus remained wedged into a slice of nacre—handy enough. Breaking the small ball further showed it to be fairly homogeneous—details upon request.

The round nucleus remained wedged into a slice of nacre—handy enough. Breaking the small ball further showed it to be fairly homogeneous—details upon request.

By now four of the eighty small golden pearls of this lot were broken to inspect the nuclei. Three of the pearls, including the above, started out with relatively organic-rich mineralization of somewhat different aspect—not unlike what you'd expect from the early nacre growth in shell.

The other pearls are yet different:

The great lighter drab ball in the dark pearl appeared covered with the familiar mineral (left), the edge of the outer nacre layer appears at the right:

Pearl

Broken yet again, the interior turned out very different indeed!

The distinct, thin, light-colored outer layer consists of patchy mineralization similar to the examples above. The gradation of brown works its way along thin prisms: that is to say, this pearl started forming not with nacre—as seen on the surface, but with the very different material that forms an outer layer between nacre and the sea in shells…. The prisms (white rods in dark organic sheaths in the images below) here are smaller than the ones in the shell—not unheard of, but quite well formed, without interruptions—as they almost always are in shell and nearly never in my natural prismatic pearls.

The distinct, thin, light-colored outer layer consists of patchy mineralization similar to the examples above. The gradation of brown works its way along thin prisms: that is to say, this pearl started forming not with nacre—as seen on the surface, but with the very different material that forms an outer layer between nacre and the sea in shells…. The prisms (white rods in dark organic sheaths in the images below) here are smaller than the ones in the shell—not unheard of, but quite well formed, without interruptions—as they almost always are in shell and nearly never in my natural prismatic pearls.

The first image below shows the center of the pearl with two close-ups—one on the very center, showing the mineral blocks that appear to be the first material deposited toward this pearl, and the other of the end of prisms, beneath the layer with round mineral patches.

Pearl
Pearl
Pearl

This is the first pearl containing both shell materials (nacre and prismatic calcite) I have from Pteria sterna and the first with a nice, complete nacre cover. My other examples—from Bahraini Pinctada radiata, Pinna—have partial nacre covers. These are somewhat famous examples: that a single pearl could contain both shell layers was thought as a nice clue toward the origin of pearls for quite some time, if ever for different reasons…. It was as easy to check a century ago, as today, that the succession of materials in the shells was reproduced correctly da capo in shell repairs and in this kind of pearls—a fairly strong argument that natural pearls proceed not unlike shell repairs—if in the wrong place. The guess was rough, scandalously bold and impossible to verify back then. The possibility is less surprising, still seriously difficult to probe these days....

The white pearl—mentioned first in the caption of the first image above—also broke apart in stages: a thickish layer became detached off a large core of even brighter nacre that does not quite appear as such in that shot…. Interestingly, this core is made of somewhat modified nacre—thinner layers than the typical shell nacre, with tablet boundaries somewhat difficult to see… yet all the nicer! With such variations accounted for, the pearl is virtually all nacre, except for a .5 mm centre of mass of less structured mineralization much richer in organic material than nacre:

Pearl

There is more where these came from, sure enough.

Except for the pearls with diatoms inside, all appear to have started much like the respective layers of shell (nacre or prismatic) do—taking more or less space to order themselves into the respective classical shell material. The old idea that natural pearls start forming just like shell in response to some (misplaced) internal trigger, may now stand on a diverse collection of cases…. I am left expecting that subtle changes in the enormous task of shell building would be quite enough to get pearls going: no invaders, no great disasters called for. Finding which unremarkable events in a shellmaker’s life yields pearls will take greater luck than was ever called for to find the worldly remains of critters caught in pearls. It could have a name before it is known—if only to keep in mind that the prize of the hunt is not a bug, this time.

References

  1. Awaji, Masahiko, and Tohru Suzuki. "The Pattern of Cell Proliferation during Pearl Sac Formation in the Pearl Oyster." Fisheries science 61.5 (1995): 747751.
  2. Awaji, Masahiko, and Akira Machii. "Fundamental Studies on In Vivo and In Vitro Pearl Formation: Contribution of Outer Epithelial Cells of Pearl Oyster Mantle and Pearl Sacs." (2011).