At the end of the day, pearls are stone. These two look the part more than most in my collection of samples: a British oyster turned particularly heavy with mineral overgrowth including two pearls—a classically one-sided blister raised up like a finger next to the muscle scar, and a round one encased under translucent layers of calcite. Of oyster pearls, read on...
‘Pearl oysters’ is wrong, ‘oyster pearls’ are right…
More often than not I might call all molluscs that produce pearls 'pearl-oysters'. This is wrong: only some molluscs could possibly be considered 'oysters', and many more do make pearls. Still, try as I might, the correct 'pearl-molluscs' doesn’t roll off the tongue. To right the wrong somewhat, I might claim that the word 'oysters' in 'pearl-oysters' is hinting at the order (Ostreida)—not at its often iced members (Ostrea edulis). The order contains a rather large list of known pearl makers, including the historical pinnacles; it still excludes the wealth of flamed pearls mostly made by some snail or other. To name all pearl-makers, there is no escaping the awkward 'pearl-molluscs'… If there is better nomenclature, I am happy to pick it up.
In fact, the list of pearl-makers is unknown. Even one of the known pearl-making molluscs does not seem to exist. This would have some value: I trust that the biology of natural pearl-making is not understood well enough to start guessing whether some molluscs may not be able to grow rounded pebbles at least as nice as their shell. The best possible method is to count known cases…
The outliers are intriguing. For once, I hear that species may have lost the habit of shell-making, but not the capability; some individuals might still produce vestigial shells and pearls. Much more surprising to me is the apparent absence of pearls from dry land-dwelling molluscs: enough snails are harvested and handled to assume that if there are pearls they'd be found—yet, nada. Such scarcity would fit well with an interesting argument, relating pearl-making to the calcium storage: freshwater bivalves do hoard calcium and may sustain many more pearl grafts than their saltwater relatives. Another group managing Ca storage differently while making no pearls would add a third, agreeing, case. What more… With or without land snails, I rather expect that a list of pearl-making molluscs ought to exist. Be it as an endless project of—encyclopedic—conchology or pearl gemology. It’d take either village! Perhaps both.
As for oyster pearls:
There are two kinds on my tentative list: the rocky kind in the front picture, and one ~3 millimeter window-pane oyster pearl. I do not quite know which of the Placuna is responsible, but would certainly wish to see more of the kind. The shells are obviously quite beautiful—with their foliated calcite often compared with nacre.  My small windowpane pearl looks like ice—and should get better photographic treatment than this:
Above: Bahraini foliated calcite pearl on a microscope stand (next to a prismatic calcite; one from the famous Pinctada radiata). Below: a close view of the calcite sheets making up this pearl—they are roughly as wide as nacre tablets and almost parallel to the surface of the pearl—or shell, two parameters that yield the nacreous look.
The heavy oyster shell with its two pearls (at the top of this post) is becoming interesting for academic reasons (unusual crystallographic planes expressed in mesocrystals), so the two pearls will be getting my attention. Meanwhile, it so happens that a fresh paper on natural pearls has just caught up with this kind of Oyster pearls ; adding detail to last year news  telling of greater variety of pearls from the same species. If you take a look, Zwaan et al. find one aragonite pearl to have come from this calcite shell with an aragonite muscle scar—not unlike my samples received from David Leblanc's false-jingles. Lighting strikes twice!
The windowpane pearls remain a personal interest… Historical pearling books mention windowpane pearls fished for any other reason but their beauty —I may disagree.
- Checa, Antonio G., Francisco J. Esteban-Delgado, and Alejandro B. Rodríguez-Navarro. "Crystallographic structure of the foliated calcite of bivalves." Journal of structural biology 157.2 (2007): 393-402.
- Batista, Frederico M., et al. "Occurrence and characterization of pearls from oysters of the genus Crassostrea." Journal of the Marine Biological Association of the United Kingdom (2016): 1-6.
- Zwaan, JC Hanco, and Peter Groenenboom. "Natural Pearls from Edible 'True Oysters' in Zeeland, The Netherlands." The Journal of Gemmology 34.2 (2014): 150.
- Jameson writes on “Pearls from Placuna placenta from Lake Tainixdakamam, Ceylon”: 345.