Tall tale, small pearls:
Seeing is believing. These are green pearls, therefore, I am inclined to believe that natural pearls of this color do exist. Of course, there is the green iridescence of some black pearls—striking indeed, but no cigar... Perhaps, somewhere, somehow, a False-Jingle shell (Pododesmus macrochisma) is making a great one.
In my pearls, the pigment seems to blend differently in the two major materials present: it is unrecognizably diffuse through the light green calcite stacked in laths— the mineral type of which nearly all the shell is composed—and pooled in sheets among the translucent fibrous aragonite deposited in a thin patch where the strong adductor muscle that shuts the shell, gets its grip. The light green pearl—appearing barely green against the background of the shell at the top of this blog post, is made of foliated calcite. These deep green ones are made of the same material as the muscle attachment patch — extraordinarily thin, long, aragonite fibers, bundled in sheaves.
The two electron micrographs below show a shard of one of the deep green pearls:
Left: Aragonite fiber bundles start at the center of a green, glass-like translucent pearl roughly 1 millimeter across; apologies for the shards… Some bundles continue uninterrupted until the surface of the pearl, others are capped by layers of organic material. These areas are the green-est, but the pigment may nor be confined to these dark deposits: since all shell / pearl materials do incorporate a small organic fraction [how & what for adds up to much of their research interest], it is quite likely that greenness pervades.
In such images the near-white areas are mostly calcium carbonate—here, aragonite that may still contain a few percentages of organic inclusions very closely packed throughout the mineral, the dark material is mainly organic. Although I have since found fine aragonite fibers in a few natural pearl nuclei of several species, these bundles are among the nicer and thinnest I have almost seen. Where "almost" hedges for some limits to this rough, exploratory microscopy: try as we might, the closest we got to discerning single fibers was against the dark background of organic deposits, where the bundles of fibers start, and even then—see below.
You may have already guessed that I am quite enthusiastic about this type of aragonite—the material is by all means rare among all shells despite the disconcertingly grand array of possibilities…. Finding pearls made of it has certainly surprised me. Transparent, minute fibers like these could play with light—if they could ever get aligned in the right way, along rather than against the surface of pearls; a dream!
Calcite laths, aragonite fibers…these bring me to the tall tale: It is said—by Jameson and others writing of "muscle pearls" and elsewhere in the less academic literature on pearling, that fine natural pearls were found next to the attachment of adductor muscles to the shell. Much speculation ensued around the matter: could it be the case that the slight displacement of these muscles caused natural pearls to form…? Some of the pearls concerned reportedly contained more or less substantial cores of some other material but nacre. Indeed, there is a thin patch of some other material deposited beneath the muscle. Coincidence? It would not be easy to tell, unless muscle-attachment material were found in pearls—and it is not easy to identify which is what: it might be easy—if there are no essential changes to the material formed as pearls rather than as muscle attachment patches (luck required), else you'd need some recognizable component rather than straightforward aspect. This is exactly what the Pododesmus offered: it so happens that their muscle attachment is the only aragonite produced in a wholly calcite shell. I hear that indeed, larger calcite pearls do grow around small aragonite cores such as these…not unlike the tall tale claimed that later nacre growth may come over the muscle pearls (likely of prismatic aragonite—a catch-all name for a great many versions of mineralization).
The old tale of muscle pearls has a long paper record alongside the historical wrangle around natural pearl nucleation, with ever more evidence of "muscle pearls" acquiring nacre covers later. I am keeping a file open.
A further call for samples - of 'Podo' pearls of all kinds, preferably in their sacks, has been out since taking these shots. I have received a couple of the light green kind. More are expected.
As for the briefly above-mentioned foliated calcite pearls—they are certainly not boring. Rather the contrary. I should catch up with them soon, not in the least because some foliated calcite rivals the beloved brightness of nacre, and, for once, I do not have to guess that fine pearls made of it might exist….
Jameson, Henry Lyster, "On the origin of pearls," Proceedings of the Zoological Society of London, 1902: 140–166. (archived here)