The Oyster and the Environment
Every characteristic of the Tahitian pearl depends on the magical
qualities of the Pinctada margaritifera cumingi. Everything
that makes the Tahitian pearl unique among gems--the size of the pearls,
the colors, the mirror-like shine, the rainbow play of light--is the
product of a perfect and precise combination of factors that exist nowhere
else in the world. It takes a black-lipped pearl oyster living in the
perfect waters of the South Pacific atolls, and the skill and attention of
the people who care for the pearls.The Tahitian pearl oyster
is delicate, and sensitive to any kind of pollution, and cannot be raised
in crowded conditions. It can only be farmed in lagoons that meet
their unique climatic, biological and ecological needs.
The large size of the oyster allows for the
gestation of large pearls. The gleaming colors are unique to this
species of oyster, and the rich nuances of light stem from the interaction
of the oyster and the nutrients and temperature of the water. The
Gambier Islands of the Mangareva Archipelago in particular are renowned
for exquisite peacock green pearls. The temperature of the water dictates both the
thickness and the brilliance of the pearl Warm waters promote to the
production of layers of pearl, whereas cooler water makes for finer and
thus more transparent layers.
The Pinctada margaritifera prefers the
warm waters of the Pacific. Around the Gambier Islands, the Pinctada produces
ultra-fine layers of aragonite. The archipelago is situated to the south
of Tahiti where the water is cooler, so that the pearls acquire a
particularly fine luster and orient.
Aragonite Aragonite is a variety of
calcium carbonate (CaCO3) secreted by the mantle tissue of
mollusks. It can also be found in carbonated metamorphous rock. In
the pearl oyster, it is the substance that makes up the inner lining of
the shell, or the "mother-of-pearl", as well as the pearl
itself.
The crystal structure of aragonite is
orthorhombic, meaning it has three triangular sides, which act as tiny
prisms. It is the interaction of light with these tiny prisms in the
pearl that create the quality referred to as orient.
Pinctada margaritefera is capable of
secreting three or four layers of aragonite each day. This means that,
over a two-year life cycle, some 2000 layers will have been deposited,
each one about one micron (0.001 mm) thick.
Reflection, refraction and diffraction
transform the light as it passes through the layers of aragonite A Tahitian pearl has many
thousands of layers of aragonite, depending on the length of time the
pearl spends in the oyster. Each one is extremely thin and has the
appearance of a transparent film.
- Reflection: light bouncing off the surface
of the pearl
The surface of the pearl acts like a mirror. The smoother the
surface, the brighter the pearl will appear. This smoothness is
part of what gives Tahitian pearls their superior luster.
- Refraction: light passing through the pearl
The transparent aragonite acts like a
prism, bending the light as it passes through the pearl. Some of
the light bounces off the layers of aragonite. The more transparent
these layers, the easier it is for the light to pass through and reach
deeper into the pearl.
- Reflection and refraction combined
produces "interference", causing the pearl to radiate
iridescent light
The layers of aragonite work as a
"diffraction grating", breaking the light into a shimmering
spectrum. This is the effect known as "orient",
the moving rainbow of colors so remarkable in Tahitian pearls.
Interference increases surface brightness,
luster, inner luminosity and orient, giving the impression of a glowing,
translucent sphere.
The larger the pearl, the more these light
phenomena are pronounced. If the layer is too thin,
the light tends to be diffused rather than reflected resulting in poorer
luster and orient. On the other hand, the more layers of aragonite film
there are, the larger the pearl and the more exquisite its luminous
qualities to behold.
The largest perfect Tahitian cultured pearl in
the world is known as the Robert Wan Pearl. It measures 20.92mm in
diameter (over 13/16 inch) and weighs 12.5 grams, and was harvested in May
1996 at Robert Wan's pearl farm at Nengo Nengo atoll in the Tuamotu
Archipelago.
Quality Criteria Scientists who are experts
in the field of gemology have defined the same quality criteria for all
types of pearls, which are recognized by all professional jewelers.
- Size: The larger the pearls diameter,
the rarer it is. South Sea pearls, including those from Tahiti,
Australia and the Philippines, measure between 9 mm and 14 mm. Chinese
and Japanese pearls measure between 2 mm and 9.5 mm. And there are
some extremely rare pearls with a diameter as big as 18 mm.
- Shape: There are four official shapes:
- Round:These are considered the most
beautiful pearls. They are either perfectly spherical or with
diameters varying less than 2%.
- Semi-Round: Such
a pearl has a diameter varying no more than between 2-5%.
- Semi-Baroque: This
pearl has a regular shape with at least a rotation axis and applying
to four different shapes: drop, pear, oval and button.
Baroque.
This covers all other forms of pearls, which
generally have an irregular shape and do not have any axis of symmetry.
There also is a sub-category of semi-baroque pearls, known as ringed, or
circled, pearls. They are characterized by regular streaks or rings,
perpendicular to an axis of symmetry, over more than a third of the
pearls surface.
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| Round |
Semi-Round |
Semi-Baroque |
Baroque |
Circled |
- Official pearl surface quality
definitions
- "A" quality: Any pearl with
imperfections on less than 10% of its surface, but having a good
luster.
- "B" quality: Any pearl with
imperfections on less than a third of its surface, but having an
average luster.
"C" quality: Any pearl with imperfections on less than
two-thirds of its surface, but having an average luster.
"D" quality: Any pearl with imperfections on more than
two-thirds of its surface, or a pearl with no luster at all.
- A pearls brilliance depends
on both its luster and its orient, or a combination of reflected,
refracted and diffused light. A pearls luster is a measure of the way
its surface appears to shine, ranging from matte to very glossy. The
proper luster of a Tahiti cultured pearl must have a reflection of
light that is nearly perfect. A pearl with good luster has layers of
nacre that are even and thick, reflecting light like a mirror. Thus, a
pearls luster is its surface brightness as produced by reflected
light.
Orient, the inner light that radiates from
within a pearl and determines its iridescence is another subjective
factor used in judging the quality of a Tahiti cultured pearl. Orient is
the pearls other luminous property: the ability to absorb rays of light
through various layers of aragonite and emanating such light from
within. Thus, orient is the light from within the pearl as created by
the combination of reflection, refraction and diffusion.
- Surface purity. As
a living organism born in the lagoon, each pearl has unique markings
that only nature could bestow: tiny outgrowths, or depressions,
referred to as pitting, and faint shadows or colored areas. Thus,
pearls with the greatest amount of purity have the fewest defects, if
any at all.
- Pearl layer thickness around the nucleus.
This criteria for evaluating a pearl stems from
new pearl production conditions. Water pollution, together with a
tendency to give priority to short-term profitability, are, of course,
incompatible with the slow process involved in building up successive
layers of aragonite around a pearl's nucleus. That is why the Pearl
Science Laboratory of Japan established the following pearl thickness
guidelines:
- 0.8mm of nacre for a pearl of outstanding
quality;
- 0.4mm-0.6mm, or 19 months of continuous
growth, for a pearl of fine quality;
- less than 0.3mm, or 7-8 months of growth,
for a so-called inferior quality pearl;
- And, less than 0.2mm, or 3-6 months of
growth, for an unacceptable pearl, more often known as a
"reject".
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