How does an oyster make a pearl?
Contrary to popular belief, a natural pearls is not
formed by a grain of sand. This story is an urban legend.
A natural pearl forms inside a bivalve mollusk when the shell
has been invaded by a parasite or damaged. The most common reason a
natural pearl will develop is due to parasitic invasion.
The parasite wil burrow through the periostracum layer (outer
layer) of a mollusk shell and into the mother-of-pearl beneath.
When the parasite penetrates the mother of pearl in comes in
contact with the mollusk's mantle muscle. This muscle contains
cells known as epithelial cells. These cells produce a substance
called nacre which coats the inside of the shell (mother-of-pearl)
and is the substance of which a pearl is composed.
The parasite becomes lodged in this mantle muscle and dies. The
mollusk's definsive reaction is to grow a sac around the intruder.
This sac is called the pearl sac. Once the sac encases the intruder
the cells begin to deposit nacre- which is composed of aragonite
and calcite (a calcium carbonate compound) platelets.
Cultured (marine) pearls are grown by inserting a rounded bead
of mother-of-pearl and a piece of mantle tissue containing
epithelial cells from a donor mollusk into the gonad, or
reproductive organ, of a host mollusk. The donor tissue grows into
a pearl sac around the bead. As in natural pearl production, this
sac then deposits nacre.
Maturity time of artificial oyester pearl