Thomas-- You can take a jewelers saw and saw around the pearl,
either close to the pearl following it’s shape or as a more free form
object. Please keep in mind that working with MOP can be dangerous–
the pearl dust is toxic, on a par with silica or other mineral dusts
in that it accumulates in lung tissue.
This “pearl” could be one of several things. Without seeing your
picture, sounds like what is known technically as a "blister"
pearl-- normally, what you see is what you get, there is nothing
inside to liberate.
If it’s a perfectly round hemisphere or other regular shape such as
a heart or drop, it’s probably a “Mabe” type blister, where a manmade
nucleus, usually a hemisphere of plastic or other material, is
cemented to the shell and subsequently coated with nacre by the
mantle tissue of the host mollusc. Your description of the
circumstances suggests Mabe-- usually $200 - $400 retail (3K)
Here you should be careful if you cut close to the pearl because the
MOP shell layers can separate off leaving an thin “eggshell” of
nacre and the nucleus which could be anything from plastic to
soapstone. The nacre doesn’t really attach to the nucleus so the
nucleus can pop out.
“Blister” pearls are also created by cementing a MOP shell bead to
the living shell, so the mollusc coats the bead with nacre much like
a typical saltwater bead nucleated cultured pearl, i.e. Akoya, South
Sea, or Tahiti. In this case, the nacre attaches to the bead much
more firmly and the nucleus is an integral part of the pearl, so you
can trim the shell however you like-- some times these are made into
3/4 pearls with a flat spot on the bottom.
Natural blister pearls are caused where various natural actions like
parasitic invasions or pieces of shell debris become the nucleating
agent. These would characteristically be irregular in shape.
Rarely, a pearl (within the body of the mollusc) will get so big, it
can break thru the mantle tissue and settle against the shell. The
mantle will repair the break and the natural nacre secreting action
will continue and begin to form a “blister”. Old time AMFW pearlers
would look for “crippled” shells-- the blister would cause irregular
shell growth. This was a case where a true natural pearl could be
"liberated" – this required a “Pearl Doctor” who knew the difficult
technique known as “peeling” – truly a lost art these days.