The Culturing Process and Further Working
The rather muscular inner soft body of Pteria penguin limits the production of cultured blister pearls. A rounded or halp-spherical nucleus is glued onto the inside of each shell (which means that two pearls can be produced in one animal). The nucleus is sometimes less rounded than a half-sphere and sometimes more rounded, in which case it may take the shape of a three-quarter sphere. The color and appearance of the future pearl depend on the position of the nucleus inside of the shell, which must be sufficiently far away from the black outer rim but must not disturb the inner soft body.
The growth period amounts to two years, although it was three years in the beginning. The growth rate is low, only 0.7mm to 1.5mm per year, and varies with the season.
After the dome has been covered by a suitable thickness of nacre, the whole is cut from the shell. The dome itself is removed: steatite is used in particular for its non-adherent qualities. Then the hollow nacre shape may be filled with a resin and the base then covered by a polished mother of pearl backing piece. The diagram shows one typical shape for the backing; many composite pearls have flattened or slightly domed backs. Mabe pearls range from 9mm to 17mm or more and hear and drop shapes are commonly produced.