Windowpane Pearls: Calcitic nacre?

[SteveM]

A contact in Indonesia has shown me a parcel of pearls from the Placuna Placenta, family Placunidae, popularly known as the 'Windowpane Oyster' for its translucent shell traditionally used as a stand-in for glass in its native breeding areas. As can be seen, the pearls appear distinctly nacreous. But they contain no aragonite whatsoever, being composed of 99% calcite and 1% organic material, accounting for color. GIA only began studying and identifiying these pearls in 2023.

Nacreous-appearing calcite has implications for other threads in this forum that have debated the definition of nacre and the significance of aragonite in the classification of pearl qualiity.

Another intriguing aspect of these pearls is that they flouresce red under UV, exactly the same as Pteria Sterna (Sea of Cortez), thus throwing a wrench into Pteria dogma as well.

Windowpane pearls appear to be uniformly small in size—think Poe Pipi—but with similar attractiveness and as worthy of jewelry application.

A new chapter for the pearl books?

 

[SteveM]

Nacreous-appearing calcite has implications for other threads in this forum that have debated the definition of nacre and the significance of aragonite in the classification of pearl qualiity.

Specifically, that the definition of 'nacre' as the iridescent result of stacked aragonite platelets is too narrow. It must rather refer to the smooth interface between shell material and mollusk. Whether iridescent, porcelanous or (in this case) metallic in appearance becomes irrelevant.

@LagoonIslandPearls argued this point to my puzzlement at the time.

 

[CortezPearls]

Find this intriguing...do you have a Placuna shell with you? Would it be possible to use your UV lamp on it?
If it does glow pink-red then this genus also has the same -or similar- porphyrins in its shell...if not: these pearls could belong to a Pteria species and not Placuna.

 

[SteveM]

Find this intriguing...do you have a Placuna shell with you? Would it be possible to use your UV lamp on it?
If it does glow pink-red then this genus also has the same -or similar- porphyrins in its shell...if not: these pearls could belong to a Pteria species and not Placuna.

Just seeing this. The GIA report linked above does include images of Placuna pearls as well as shells under UV.

 

[CortezPearls]

Just seeing this. The GIA report linked above does include images of Placuna pearls as well as shells under UV.

Got it! Just downloaded the file...I was traveling and with very baaaaad internet!

 

[SteveM]

Find this intriguing...do you have a Placuna shell with you? Would it be possible to use your UV lamp on it?
If it does glow pink-red then this genus also has the same -or similar- porphyrins in its shell...if not: these pearls could belong to a Pteria species and not Placuna.

Nothing at all on the internet to confirm porphyrin pigment in Placuna Placenta. But at just 1% organic shell composition it wouldn't be much.

 

[SteveM]

Attaching detailed studies of Placuna Placenta: 2012 on the microstructural and optical qualities of the shell and 2022 on comparative genetics. Somewhere in this forest of data one would hope something pertaining to this species' red UV shell flourescence might be found.

The SEM images do show the calcite microstructure as a layered laminate, supposedly capable of approximating the optics of terraced aragonite platelets (nacre).