How do oysters make pearls? - Rob Ulrich
While most people wouldn’t consider
the crusty exterior of an oyster
to be particularly beautiful,
opening up this craggy case might reveal
an exquisite jewel nestled within.
Yet, despite their iridescent colors
and smooth shapes,
pearls are actually made
of the exact same material
as the shell that surrounds them.
Pearls, urchin spines, the shells
of mussels, snails and clams, even coral—
all these structures are made
out of the same chemical compound:
calcium carbonate.
So, how does this single ingredient form
such a vast array of materials?
Calcium carbonate, or CaCO3,
is common on land,
and even more bountiful in the sea.
The Earth’s crust is rich in calcium,
and over millennia these deposits
have seeped into rivers and oceans.
This is especially true
near hydrothermal vents,
where hot seawater mingles
with calcium rich basalts.
Meanwhile, when carbon dioxide in the air
interacts with seawater
it eventually produces
dissolved carbonate.
Every year, the ocean absorbs roughly
one third of our carbon dioxide emissions,
adding huge quantities of carbonate
into the water.
It’s no surprise that sea creatures have
made use of these abundant compounds,
but the way calcium and carbonate are
woven together into various shapes
is surprisingly artful.
Let’s return to the humble oyster.
Like many aquatic mollusks,
oysters start life as exposed larvae,
and quickly get to work building
a protective shell.
First, an organ called the mantle secretes
an organic matrix
of proteins and other molecules
to construct a scaffold.
Then, the oyster filters the seawater,
drawing out calcium and carbonate to
combine them into its building material.
It lays this material over the scaffold,
which is covered in charged proteins
that attract and guide
the calcium carbonate molecules
into layers.
The specific arrangement of these protein
scaffolds depends on the mollusk species
and their environment,
accounting for their vast diversity
of shell shapes, sizes, and colors.
Mollusks carefully control all components
of their calcium carbonate creations—
even manipulating CaCO3
at the molecular level.
Using special proteins,
mollusks can produce
two crystal structures out of CaCO3:
calcite and aragonite.
Both of these compounds have
the same chemical composition,
but different qualities due to the way
their crystal lattices are arranged.
Calcite is the more stable of the two
and less prone to dissolving over time,
so most mollusk shells have
a sturdy outer layer of calcite.
As the slightly more soluble molecule,
aragonite can better adapt
to more or less acidic environments.
So most mollusk shells have
an interior layer of aragonite
to maintain their internal pH level.
But one form of aragonite is stronger
and more versatile than the rest:
nacre.
Mollusks make this special material
by placing successive layers of aragonite
interspersed with proteins.
These layers are stacked
like hexagonal bricks,
each surrounded by other organic material
that directs their orientation.
The uniform layering and brick-like
structure of nacre
is key to its signature iridescence.
The layers are similar in thickness
to the wavelength of visible light,
so the light reflecting
from its interior surface
interferes with the light reflecting
from the outer surface.
When particles of light strike the nacre,
they bounce around its multilayered
crystalline structure
in a cascade of shifting rainbows.
But nacre isn’t just pretty—
it’s one of the strongest and lightest
biomaterials we know of.
And it's not just oysters that produce it.
In fact, numerous mollusk
species deploy nacre
as one of their primary
defense mechanisms.
If an intruding parasite or even a stray
particle of sand irritates the mantle,
the mollusk will coat the offender
in nacre-producing cells
to form what’s known as a pearl sac.
These cells wrap the threat in layers
of proteins and aragonite
until eventually the cocoon
completely absorbs the invader—
dissolving the threat into an opalescent
sphere of nacre.
This defense mechanism is our leading
theory for mollusks making pearls;
transforming everyday intruders
into timeless treasures.
