"Mabe pearl" is one of those trade terms that hides a more precise reality, and gemologists are careful about it for good reason. A true pearl — natural or cultured — is a rounded growth of biomineralized aragonite and calcite, bound by the organic protein conchiolin, formed inside a pearl sac within the mollusc. A cultured pearl is the same thing, simply started by human intervention. A mabe is different in kind: it does not form free inside a pearl sac at all.

Instead, a mabe begins as a cultured blister — a dome grown against the nacreous inner wall of the shell, over a hemispherical nucleus glued to that wall. After harvest it is heavily worked: cut from the shell, the soft nucleus removed, the hollow filled with a hardening compound, and a mother-of-pearl cap glued over the base. The finished piece is therefore an assembled product, most accurately called an assembled cultured blister rather than a pearl. That is why it has a flat back and shows only its glossy dome — and why it costs less than a comparable full round pearl while looking large and luminous face-up.

The name itself is a clue to the origin. "Mabe" comes from the Japanese mabe-gai, the vernacular for Pteria penguin, the winged pearl oyster long used to grow these blisters. Other species are used too, including the rainbow-lipped oyster Pteria sterna ("ostra nácar") of the Gulf of California, whose blisters carry striking natural color.

The technical description above follows the gemological account of Rui Galopim de Carvalho, illustrated in his post with bead-cultured pearls, an assortment of cultured blisters, a pendant, and one blister still attached to a Pteria sterna shell from Guaymas, Gulf of California, Mexico © Perlas del Mar de Cortez.

Source: Rui Galopim de Carvalho / Portugal Gemas Academy

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