Gem of the Month
Each month this section will feature
either a topic of interest to gem lovers or one special gemstone with
background on the material and its value.
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October, 2005
Natural
Pearls
Prior to the 19th century, when they
were superseded in price by diamonds, natural pearls had, throughout
history, been valued above all other gems. Although their beauty, and
the fact that they come out of the mollusk ready to use, were
important factors, it was sheer rarity that drove their value to the
highest levels. The formation of a large, beautiful and perfect
natural pearl is an event so
unlikely in Nature that only those at the
pinnacle of wealth and power in a society were able to own them.
Depending on the species between 1/1000 to 1/500,000 mollusks will
form pearls during their lifespan, and the vast majority of those
formed will be small, off-color or flawed.
Nacreous Pearls
True pearls are referred to as
"nacreous pearls" due to their composition. Such a pearl is formed
when a small foreign object makes its way into the soft body of a
filter-feeding or grazing mollusk, and cannot be expelled. The
irritant is sometimes a stray bit of shell or bone, but more often a
parasite. Layers of calcium carbonate crystals and protein are
secreted to slowly cover all parts of the object which, then, becomes
a pearl. If it grows entirely within the body of the animal it will
be three-dimensional "cyst pearl", if it grows attached to the shell,
it will be a "blister pearl".
We can perhaps imagine the awe and
mystery that these objects held for the early peoples who found and
cherished them, and it is no wonder that mythic and mystical
explanations for their formation abounded. Picture harvesting the
creature shown below (not all pristine and clean, as shown), but
covered with mud and sea weed and inside its greyish, lumpy and slimy
interior, finding the iridescent object in the adjacent photo.
[Abalone shell,
abalone pearl jewelry: Images courtesy of
www.allnaturalpearls.com]
The beauty of a nacreous pearl comes
from a combination of its shape, color, and surface reflection
(luster). In the best specimens these features are heightened by a
surface iridescence called "orient". The shape of a pearl will
largely be determined by chance (the shape of the irritant), and its
anatomical placement in the animal. The body color will vary with the
species of mollusk, which will generally make pearls in shades
similar to that of their shell lining. The iridescence and/or luster
of a pearl will be a consequence of the perfection and thickness of
the nacre layers, of which the onion-like pearl is made.
Nacre (NAY-ker) is made up of
plate-like hexagonal crystals of translucent aragonite (a form of
calcium carbonate), conchiolin protien (konk-KY-oh-lin) and water.
Each crystal is very thin and they are layed down rather like bricks
in staggered courses with the proteinaceous "motar" between. Light,
reflecting and diffracting from the uneven surface, and the thin
inner layers creates the lovely effects of luster and
orient.
[Magnified surface
of a pearl with its overlapping layers of nacre: Image courtesy of
Joe Mirsky]
Pearls are made by both salt and
freshwater bivalve (two shell: oysters and mussels) and univalve (one
shell: snail-like) mollusks. What we call saltwater pearl "oysters",
are not closely related to the edible varieties of oyster. So, that
plate of oysters you might enjoy for lunch, must remain a gastronomic
delight alone, as no pearl will be found in it. If you are a
freshwater mussel or saltwater abalone fan, however, your chance of
finding a pearl, though slim, exists, as long as you eat them raw
(cooking destroys pearls).
What's in a
name?
In today's world, the word "pearl"
means "cultured pearl" to almost everyone. Technically, it's illegal
to sell or advertise cultured pearls, as pearls, without using the
adjective "cultured", but no one really gets very excited about
enforcing it. In the present day, natural pearls, due to overfishing
and pollution, are even rarer than they were in
historical times. Their admirers are not so much the wealthiest among
us, as those who for philosophical, spiritual/religious, or aesthetic
reasons seek out these rarities. This shift in the position of
natural pearls from status symbol to quasi-cult objects, has been the
result of the spectacular success of pearl culturing. With its
technical beginnings around 1890 and large scale production in force
by the early 1920's pearl culturing has made this gem one that is
obtainable by virtually anyone in a variety of qualities and
prices.
How the pearl
marketplace has changed, then:
[Circa 1890 2-3.5
mm, natural, saltwater pearl brooch, circa 1920 2.5 - 4 mm, natural
saltwater 16" pearl necklace]
Natural Pearls:
Expensive then, expensive now!
And now:
[Circa 1960 4 mm,
cultured saltwater pearl brooch (fairly expensive when new), 2005,
18" 7.5 mm, freshwater cultured pearl necklace (very
inexpensive)]
Cultured Pearls:
Size is increasing and prices are dropping!
In looking at the photos above we can
begin to see why cultured pearls have taken over the world. Natural
pearls are generally small, and vary in shape and color, cultured
pearls, especially in today's market, are uniform in size, shape, and
color and can be huge! It might take an oyster in Nature six or seven years to
make a 4 mm pearl, and only 1-2 out of a hundred of them would be
round. Furthermore, colors are not uniform.
In today's market natural pearls are
available through current small scale (legal) harvesting, and also as
vintage and antique specimens or jewelry.
Examples of natural, nacreous
pearls:
[A contemporary
suite of saltwater pearls from the "Rainbow-lipped" oyster: Image
courtesy of www.allnaturalpearls.com, Victorian Era seedpearl bride's
necklace, Art Deco Era saltwater pearl necklace, contemporary suite
of abalone pearls: Image courtesy of www.allnaturalpearls.com, circa
1905 gentleman's saltwater pearl stickpin]
Are they really
natural?
If you're in the market
for antique jewelry with natural pearls you need to be aware that
imitation pearls have a long
history and were sometimes used in surprisingly "upscale" jewelry
pieces. The "tooth-test" is generally helpful (although not
sanitary). The surface of a nacreous pearl (natural or cultured) will
be slightly gritty. This microscopic roughness can be detected by
rubbing the pearl, gently, across the edges of your front teeth.
Pearl simulants (usually made of glass or shell) have smooth surfaces
and don't feel rough. Also if the pearls in your antique piece are
uniform in size, luster and color your "fraud antennae" should be on
full alert.
Non-nacreous
"pearls"
Technically, the
product of a mollusk that is not made
of nacre is not a
pearl, but a "calcareous concretion". Having said that, I'll point
out that they run the gamut from chalky marble like products such as
found in edible oysters with no gem value, to some of the most highly
valued and rare gems in the world. For the purposes of this essay, I
will call these beautiful and valuable ones, "pearls".
There are three of
note: Conch pearls (pronounced "konk"), scallop pearls, and melo melo
pearls. Each of these is made of calcium carbonate but primarily in
the form of calcite rather than aragonite, and with different
structural characteristics and protein proportions than their
nacreous cousins.
Conch pearls are
products of a large marine snail, the queen conch, It is native to
the Caribbean and, until it was fished to near extinction, was found
abundantly in the waters of the Florida Keys. Ranging in color from
white to vibrant pink, the pearls are usually small (8 mm is large)
and ovoid. you can see in the picture below and to the right, the
highly desirable "flame structure" chatoyance that the best specimens
have. (Conch pearl lovers are not to blame for the decimation of the
Florida population, as they are basically just a rare by-product of
the hunt for this mollusk: the meat of the conch is a delicacy, its
pink shell lining is used in jewelry, especially cameos and the shell
itself is a tourist object.)
[Strombus gigas, the Queen conch, with typical pearls: Image
courtesy of www.sunlion.com, three top quality conch pearls showing
"flame structure": Image courtesy of Aires Jewelers, an attractive
and protective contemporary 14k setting for a small conch pearl:
Image courtesy of www.faisongems.com]
Scallop pearls
The newest type of
natural pearl available to collectors is the scallop pearl. It is
found in a marine bivalve scallop that is native to the coast of Baja
California, and is just beginning to be harvested. Highly variable in
size and shape, they have mosaic-like patterns and cream to salmon or
mauve colors with a semi-metallic to chatoyant sheen.
[Scallop pearls:
Images courtesy of www.allnaturalpearls.com]
Melo melo
pearls:
By far, the hardest to
obtain pearls on Earth are those of the marine "baler" snail, found
in the Indo-Pacific region: round, smooth and sometimes quite large,
I recently held one in my hand that was the size of a large gumball
and nearly dropped it when the price of $50,000 was quoted to me. The
colors and flame structure are similar to those of the conch
pearl.
[Image courtesy of
the Latendresse family]
To this date, none of
the non-nacreous pearls have been successfully cultured, and each has
a unique structure which makes its difficult to fake, so that there
is little worry about synthetics and simulants.
Value
It is difficut to talk, except in
generalities, about value for gemstones as rare and variable as
natural pearls. The nacreous ones increase exponentially in value
with size but luster, color and shape are important as well. With the
non-nacreous pearls, color and quality of the any surface pattern or
chatoyance are probably more important than size.
Care
All pearls need gentle care. They are
soft, fragile, and are sensitive to chemicals, especially acids. All
the cleaning they ever need, is wiping with a damp cloth after each
wearing, and they should be stored away from other gems, preferably
in a cloth bag or their own case. Non-nacreous pearls should not be
exposed to bright light for extended periods as the organic pigments
which give them their colors can fade. Pearl strands that are worn
frequently should be restrung every few years.
Gemological Data
Varies with species
Stones Currently Available:
Go to: Homepage
-- what's new in faceted
gems -- what's new in
designer cabochons and gem carvings --
gem of the month --
gem of the month archive --
birthstone of the month --
key to all the codes used on the ACS
site -- definitions of
terms used on the ACS site
-- how to order
-- about ACS --
about the ACS cutters --
settings for these gems
--faceting information
-- purchase UltraTec
equipment
-- Mail to
bsmigel@cox.net