Oysters get blisters! A cultured blister pearl can range in size from 4 to 10mm. These pearls have been cultured on the inside of the shell, as opposed to the interior of the oyster i.e. within the body of its organs, their cultivation is less intrusive to the oyster. Consequently these pearls have a small flat area where they were attached to the shell. Please refer to the diagram below. Blister pearls are often used for sea water or freshwater pearl earrings.
Mabe pearls a.k.a. cultured composite pearls
Mabe or Composite Pearls are normally found in sizes from 11mm to 17mm. Mabe pearls are cultivated by attaching a resin or mother of pearl half sphere to the shell itself. Nacre is built up on to this half sphere which is later cut out of the shell, the original dome shape is removed, the fragile empty half sphere is cleaned with diluted hydrochloric acid, then filled in with resin and a mother of pearl half bead and stopper. The amount of interference by man that goes into strengthening the Mabe pearl gives rise to its other description of Composite pearl. The nacre layers in a Mabe pearl are horizontal as in mother of pearl and not concentric as in a true pearl. See Diagram below, Mabe pearls are used for earrings and brooches or pins.