What
are Natural Pearls?
Natural
pearls are nearly 100% calcium carbonate and conchiolin. It is thought that natural pearls
form under a set of accidental conditions when a microscopic intruder or
parasite enters a bivalve mollusk, and settles inside the shell. The mollusk,
being irritated by the intruder, forms a pearl sac of external mantle tissue
cells and secretes the calcium carbonate and conchiolin to cover the irritant.
This secretion process is repeated many times, thus producing a pearl. Natural
pearls come in many shapes, with perfectly
round ones being comparatively rare.
Typically, the
build-up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting
of nacre (tabular aragonite). In a pearl cross-section such as the diagram,
these two different materials can be seen. The presence of columnar calcium
carbonate rich in organic material indicates juvenile mantle tissue that formed
during the early stage of pearl development. Displaced living cells with a
well-defined task may continue to perform their function in their new location,
often resulting in a cyst. Such displacement may occur via an
injury. The fragile rim of the shell is exposed and is prone to damage and
injury. Crabs, other predators and parasites such as worm larvae may produce
traumatic attacks and cause injuries in which some external mantle tissue cells
are disconnected from their layer.
Formation
of Natural Pearls
Embedded in the
conjunctive tissue of the mantle, these cells may survive and form a small
pocket in which they continue to secrete their natural product: calcium
carbonate. The pocket is called a pearl
sac, and grows with time by cell division; in this way the pearl grows
also. The juvenile mantle tissue cells, according to their stage of growth,
produce columnar calcium carbonate, which is secreted from the inner surface of
the pearl sac. With ongoing time the external mantle cells of the pearl sac
proceed to the formation of tabular aragonite. When the transition to nacre
secretion occurs, the brown pebble becomes covered with a nacreous coating. As
this process progresses, the shell itself grows, and the pearl sac seems to
travel into the shell. However, it actually stays in its original relative
position within the mantle tissue. After a couple of years, a pearl will have
formed and the shell might be found by a lucky pearl fisher.
What
is the Value of a Natural Pearl?
Quality natural pearls
are very rare jewels. The actual
value of a natural pearl is determined in the same way as it would be for other
"precious" gems. The valuation factors include size, shape, color,
quality of surface, orient and luster.
Single, natural pearls
are often sold as a collector's item, or set as centerpieces in unique jewelry.
Very few matched strands of natural pearls exist, and those that do often sell
for hundreds of thousands of dollars. (In
1917, jeweler Pierre Cartier purchased the Fifth Avenue mansion that is now the
New York Cartier store in exchange for a matched, double strand of natural
pearls that he had been collecting for years; valued at the time at $1 million USD.)
The Great Depression
effectively slashed the value of the natural pearl, but there is no doubt that
it had been some time coming. The introduction and advance of the cultured
pearl hit the pearl industry hard; it had pearl dealers publicly disputing over
the authenticity of these new cultured pearls, and left many consumers uneasy
and confused about the much lower prices. Essentially, it damaged the image of
both natural and cultured pearls alike. By the 1950s, an era of every woman
being able to own her own pearl necklace had begun, and natural pearls were
reduced to a small, exclusive niche in the pearl industry.
Related
Article: What is a Pearl Doctor?
Sources
of Natural Pearls
Sources
of Natural Pearls. Where
are natural pearls found? Where are
natural pearls found? The answer may surprise you...yes, natural pearls can be
found almost anywhere.
Today, natural pearls are rare, very expensive and are not the
basis of any commercial fishery. Traditional and still occasional sources of natural
pearls include:
a) Saltwater pearls
from historical pearling beds in the Persian
Gulf, Gulf of Mannar between India
and Sri Lanka, off Hepu on the coast
of China, waters to the north of
Australia, the islands of Oceania,
the Gulf of Mexico, and waters
surrounding Pearl Island off the coast
of Venezuela; Freshwater pearls from Scottish
and European rivers, and the Mississippi
River and its tributaries in the USA;
b) Rare baroque
saltwater abalone pearls from
Californian and New Zealand
waters; and
c) Non-nacreous pink
pearls from the pink conch of the Caribbean. Natural nacreous pearls occur
rarely as whole pearls, and more commonly as hemispherical blister pearls.
Whole natural pearls are rarely spherical, and the thinness and evenness of
their circumferential layers of nacre determine the strength and quality of
iridescence displayed by a particular pearl. As an example, compare the
appearance and structure of moderately iridescent Pinctada (silver-lipped pearl
‘oyster’) nacre with that of highly iridescent Haliotis or abalone pearl nacre.
Traditionally, nacreous natural pearls have been valued by the
factors of rarity, size or weight, perfection of shape, colour, luster and
orient, and freedom from surface imperfections. The weight of natural pearls
has been calculated in units, termed the ‘pearl grain’ that weighs 50 mg.
Previously, natural
pearls were found in many parts of the world. Present day natural pearling is
confined mostly to seas off Bahrain. Australia also has one of the world's last remaining fleets of
pearl diving ships. Australian pearl divers dive for south sea pearl oysters to
be used in the cultured south sea pearl industry. The catch of pearl oysters is
similar to the numbers of oysters taken during the natural pearl days. Hence
significant numbers of natural pearls are still found in the Australian Indian Ocean waters from
wild oysters. X-ray examination is required to positively verify natural pearls
found today.
Pearls are rounded concretions of calcium carbonate and an
organic matrix that are secreted by cells of the outer mantle of many species
of saltwater and freshwater molluscs. While any mollusc1 is theoretically capable
of producing a pearl, nacre2 – forming molluscs principally, but not
exclusively, secrete those pearls that have long been used for personal regal
adornment.
Pearls form from the same materials and by the same processes
which secrete the shells of their host bivalves, gastropods and cephalopods. Both shells and their pearls have a
composite structure of calcium carbonate (aragonite and/or calcite), an organic
matrix (essentially of the protein conchins, formerly conchiolin and
carbohydrate) and water. Although the organic content of pearls is usually less
than 1%, it is this minor component that determines both the crystallographic
properties and orientation of the calcium carbonate that is secreted by the
mollusc, and the body color of the pearl.
In order to understand how pearls form, one must first understand
the basic biology, gross anatomy and microscopic structure of those nacre-producing
bivalve molluscs that are commonly termed either saltwater ‘pearl oysters’ or
freshwater ‘pearl mussels’.
The pearl’s color (due to pigments in the pearl’s organic
matrix and/or thickness and evenness of its constituent layers of aragonite crystals),
luster (due to light reflection from the smoothness or perfection of the pearl’s
external surface) and orient (due to its subsurface iridescence and reflectivity)
are determined by the colour, luster and orient of the inner nacreous or non-nacreous
lining of the mollusc’s shell in which the pearl grew. For example:
1) The oriental pearl
‘oyster’ Pinctada
radiata/imbricata from
the Persian Gulf and the Gulf of Mannar yields mostly smallish cream to yellowish pearls
that display subtle orient.
2) The silver- or
gold-lipped pearl ‘oyster’ (P. maxima)
from North-West Australia yields
nacreous pearls of white, silver and yellow to golden hue that also display
orient.
3) The black-lipped (P.
margaritifera) pearl ‘oyster’, from Pacific Oceania, yields grey to black
pearls that also can display a spectacular contrasting iridescence.
4) Pteria
(=Magnavicula) penguin, the black-winged pearl oyster of the tropical
waters, yield distinctively iridescent pearls that have a characteristic somewhat
metallic (brown to greyish) body colour. Pteria sterna, the rainbow wing shell from the Gulf of California,
also yields pearls with highly iridescent black to dark brown nacre.
5) Freshwater (Unio)
mussels from Scottish rivers, and the Mississippi River, yield pastel-hued highly lustrous pearls with
limited orient.
6) The univalve Haliotis,
or abalone, produces mostly baroque nacreous pearls that display strikingly
iridescent nacre.
7) The pink conch (S.
gigas) of the Caribbean
yields pink to orange ‘flame’-patterned pearls that have a non-nacreous
porcellanous surfaces.
8) The giant clam (Tridacna
gigas) of tropical waters yields white porcellanous (non-nacreous)
pearls that have little economic value.
Do
you know? How to Tell Real Pearls
from Imitation Pearls
What
and Where is the Biggest Pearl on Earth?
The Pearl of Lao Tzu (also referred to as Pearl of Lao Tze and previously
as Pearl of Allah) is the largest
known pearl in the world. The pearl was found in the Palawan sea, which
surrounds the island of Palawan in the Philippines, and was found by a Filipino
diver. It is not considered a gemstone pearl, but is instead what is known as a
"clam pearl" or "Tridacna
pearl" from a giant clam. It measures 24 centimeters in diameter
(9.45 inches) and weighs 6.4 kilograms (14.1 lb).
What
is the Value of the Biggest Pearl on Earth?
While biologists would
regard this object as a kind of pearl, gemologists regard it as a non-nacreous pearl, lacking the iridescence
of the pearls that come from saltwater pearl oysters and freshwater pearl
mussels. The interior of a giant clam has no nacre (mother of pearl); instead
it is porcellaneous, like a china plate. Because of its
great size, a giant clam can create a very large pearl, but not an iridescent,
gemlike one.
The Gemological
Institute of America (GIA) and CIBJO now simply use the term "pearl"
(or, where appropriate, the more descriptive term "non-nacreous
pearl") when referring to such items, rather than the term "calcareous concretion" and, under
U.S. Federal Trade Commission rules, various mollusc pearls may be referred to
as "pearls" without qualification.
Gemologist Michael
Steenrod in Colorado Springs has appraised the pearl at $60,000,000 (1982) and $93,000,000 (2007).
Another 1982 appraisal, by Lee Sparrow who owned a gem laboratory and appraisal
business in the Phelan Building in San Franciso, put the pearl at $42,000,000.
In America, the pearl
was exhibited at the Ripley's Believe It or Not! Odditorium in New York, valued
at $35,000,000.
The Palawan Princess,
considered the second largest pearl,
was offered at auction by Bonhams and Butterfields of Los Angeles on December
6, 2009. Though the five pound pearl was estimated to bring $300,000 to
$400,000, it was not sold.
References:
Cobb, Wilburn Dowell. 1939.
"The Pearl of Allah". Natural History.
Kunz, George F.;
Stevenson, Charles. 1908. The book of
the pearl. New York: The Century Co.
Landman, Neil H., et
al. 2001. Pearls: A Natural History.
Harry Abrams, Inc. ISBN 0-8109-4495-2
Ward, Fred. 2002. Pearls (Fred Ward Gem Book),
3rd Edition. Gem Guides Book Company. ISBN 1-887651-08-X
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