I'm not a pearl connoisseur so have no concept what is the
difference between a "pearl" and a "Keishi pearl".
It's "keshi" not "keishi". A keshi pearl is a pearl but it's
created in a very particular way. It is not cultured, per se; by
that I mean that it has not been manually nucleated. Instead it is a
byproduct of the culturing process.
In other words, a worker at a pearl farm nucleates an oyster which
then forms a pearl around that nucleus. That's a cultured pearl.
But the very same oyster simultaneously and spontaneously forms a
second (or third or more) non-nucleated pearl as well. That's a
keshi pearl. It's chemically identical to any pearl except for the
lack of a nucleus, which is the reason for the typically irregular
Pearl sellers will usually claim that keshi pearls are natural as
opposed to cultured. In one sense they're right but not according to
the GIA, if I remember correctly. The GIA considers them cultured
pearls because their existence was triggered by the culturing process
even though they were not nucleated.
Also, because they have no nucleus, they are all nacre and therefore
highly lustrous as a rule. For my money, there's no strand of pearls
more beautiful than a strand of silver grey keshi pearls.