The photo requires better resolution to have an idea of what truly is happening there...it could be "missing nacre", but the shape (almost a perfect rectangle) makes me wonder...
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.