Blister Pearl
A pearl that grew attached to the shell wall, so it is domed on one side and open on the other.
Blister pearls form when an irritant lodges between the mantle and the shell, and the mollusc seals it against the shell itself.
They are cut out with a piece of shell still attached, which is why they are flat-backed rather than whole.
A mabe is the deliberately cultured version of the same thing, using a hemispherical nucleus.
When you are buying
Judge a blister pearl on the dome only, since the back will be hidden by the setting. Check the edge where nacre meets shell, because that join is where poor examples show.
Mabe Pearl
A half-dome pearl grown against the inside of the shell rather than free in the tissue. Flat-backed, so it suits rings and pendants.
Mother of Pearl
The iridescent nacre lining of a mollusc shell. The same material as a pearl, but formed as a sheet rather than a bead.
Bezel Setting
A metal rim wrapped around the edge of a stone or pearl to hold it. The most protective setting there is.