Pearls Introduction
Transcript of Pearls Introduction
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Pearls:irritants, iridescence and industry
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Which Critters Make Pearls ?
However, pearls can also be
made by many other bivalves(e.g. mussels), as well as some
gastropods (e.g. conchs), and
even cephalopods (Nautilus ).
Basically, any mollusc thatsecretes a shell is capable of
producing a pearl, but high-
lustre (nacreous) pearls are
limited to molluscs with a
nacreous (aragonitic) layer.
The conchs and blue mussels
do not secrete nacre, so their
pearls are not nacreous.
Queen conch
(a gastropod)
Abalone
(a gastropod)
Edible blue mussel
(a bivalve)
Pen shell
(a bivalve)
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Largest Pearl (from Philippines; collected 1934)
The largest known pearl comes
from the world’s largest Giant Clam
(Tridacna gigas ).
It is known as the “Pearl of Allah” as
it was found by a Muslim diver and
though to resemble a turbaned face.
It is not nacreous.
Irregular, brain shaped, blister pearl
(hemispherical pearls attached to
shell).
The pearl measures 23 cm long and
weighs 6.35 kg (14 lbs).
The clam itself weighed 160 Ibs.
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How a Pearl Forms
It is no coincidence that the characteristics
of pearls, such as colour and lustre, match
the characteristics of the nacreous layer inthe molluscs that make them.
Nacreous pearls, like mother of pearl, are
composed of nacre and are built by the
epithelial (surface) cells of mantle tissue.
Any foreign body that irritates the mantle
tissue and cannot be expelled by the
mollusc can form the nucleus of a pearl
(the mollusc reduces irritation bysurrounding the irritating body with smooth
layers of nacre).
Rarely do grains of sand form the nucleus
of a pearl (oysters are quite efficient at
expelling sediment particles)
Cross section of natural
pearl showing layers of aragonite (separated by
layers of conchiolin).
Note that light penetrates
through the pearl, giving it a
warm glow throughout.
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Blister Pearls
The most common type of
pearls in nature are blister
pearls (pearls adhering to the
nacreous layer of the shell).
Blister pearls form when an
irritant (often a parasite)
becomes trapped between the
shell and the mantle tissue or
tries to drill through the shell
from the outside.
The oyster (or other mollusc)
simply covers over the irritant
with nacre, forming a blister.
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In this case, nacre was secreted
around a clam that managed to
bore into an abalone shell from
the outside of the shell.
Blister Pearls
In this remarkable specimen,
a fish somehow got trappedbetween the mantle and
nacreous surface of a pearl
oyster. The fish has been
covered with nacre, forming a
blister.
Prismatic layer
Nacreous layer
Nacreous layer (blue)
Mantle (grey)
Parasite/intruder Blister
pearl
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Free pearls are formed less readily
than blister pearls.
This is because the irritant must
be completely surrounded by
nacre-secreting epithelial cells of
the mantle and held away from the
nacreous layer of the shell.
Free pearls
Nacreous layer (blue)
Mantle (grey)
Free pearl
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Free Pearls
In most cases, natural free pearls
form by the intrusion of a parasite.Movement of a parasite stimulates
an invagination of the epithelium.
Epithelial tissue completely
surrounds the invader, forming a
pearl sac in deeper levels of the
mantle.
Nacre is secreted on all sides of the
invader, forming a free pearl.
Natural free pearls are formed deep
within mantle tissue or in the gonad
(if epithelial cells are moved there
by the invading parasite).
shell
(nacreous layer)
parasite
epithelial cells
of mantle
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Properties of Pearls
The same properties valued in mother of pearl
are valued in pearls: lustre, colour and orient.
As for mother of pearl, high reflectivity and
internal reflection determine the lustre of pearls.
The basic colour of a pearl (colour body) is
dependent on pigments in conchiolin (darkpearls tend to have thick layers of dark-coloured
conchiolin, whereas white pearls have thin layers
of light-coloured conchiolin). Conchiolin colour
varies among various species of pearl oysters.
As in mother of pearl, the orient (iridescence) in
a pearl is caused by the breakup of white light
into colours of the spectrum by surface relief and
the refractive/reflective properties of aragonite
crystals.
Black pearls are produced
by oysters that have a black
nacreous layer (the black
colour results from high
concentrations of black
pigment in the conchiolin)
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Fossil pearls
As the nacreous layer of shells can
sometimes be preserved in the fossil
record, so too can pearls (althoughthese are extremely rare).
These are fossil pearls of pen shells
from Eocene (50 million years old)
London Clay – they retain their nacreouslustre due to exceptional conditions of
preservation (most importantly, lack of
dissolution)
Pearls in fossil pen shell
Modern pen shell with pearls
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Cultured Pearl Industry
The practise of perliculture has greatly increased the availability of
pearls to the general public.
Wild pearl oysters have been nearly driven to extinction in Hawaii
and Tahiti. Extensive pearl farming takes the pressure off these
natural sources.
Populations of wild pearl oysters are also threatened by pollution.
Some advantages of perliculture include:
1. Better pearl count to oyster ratio
2. Some control over pearl shape3. Control over pearl size.
It is, however, a very labour-intensive industry
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The Cultured Pearl Industry : Oyster Surgery 101
Oysters, raised in cages or nets (mostly to prevent predation byother animals), are anaesthetized so that the oysters relax their
adductor muscle and open their shell.
They are now ready for tissue implant.
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Epithelial mantle tissue of
donor oysters are cut into
small strips.
In each recipient oyster, a
slice of mantle tissue, plus
a nucleation bead
(generally made from
nacre of freshwater
clams), is inserted into the
gonad (far removed from
nacreous layer of shell).
The latter ensures that the
pearl remains free
(separate from the shell
nacreous layer).
A technician cuts
epithelial mantle
tissue to be
implanted in a
cultured pearl
oyster.
A nucleationbead and a strip
of donor tissue
are inserted
in the gonad of
the pearl oyster
Shells of freshwater
mussels are cut andpolished to make nucleation
beads for cultured pearls.
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A pearl sac forms in the gonad.
The epithelial mantle tissue continuesto secrete nacre and, if all goes well,
covers the bead with nacre to form a
free pearl.
Natural pearls generally have alarge amount of nacre, relative to
the diameter of the nucleus.
Cultured pearls only have a thin
rind of nacre surrounding a larger
nucleus (the thickness of the
nacreous rind must be at least
15 % of the total diameter of the
pearl to be worth selling).Natural pearl Cultured pearl
Small nucleus
Large nucleus
(nucleation bead)
S f
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Success Rate of Perliculture
The ratio of pearls per number of oysters is higher in cultured oysters than
wild oysters, but the yield is still surprisingly low.
Under the best circumstances, out of every 1,000 oysters grown at aJapanese pearl farm:
500 die during the culturing period
250 produce poor-quality pearls
200 produce saleable pearls of low to medium quality50 produce top-grade, gem-quality pearls (so 1 out of 20 oysters).
We must assume that the surgery, presence of the nucleation bead and
close-quarters environment of the nets have a highly detrimental effect on
oyster viability. Of course those that produce high quality pearls are
generally also killed in the extraction process.
It takes about 2 years to produce a marketable pearl with a layer of nacre
about 0.4 millimetres thick (pearl size varies according to the size of the
nucleation bead inserted in the oyster). The average diameter of Japanese
pearls is about 7 millimetres.
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Major Pearl-Culturing Centres
(not to be memorized- just for general interest):
Pearl Oysters (various species)
Japan
Australia
South Sea Nations (Papua New Guinea,
Indonesia, Philippines, Thailand)French Polynesia (e.g. Tahiti)
Mexico
Freshwater Clams (various species)
ChinaJapan
Thailand
India
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Mabé Pearls
A fairly new type of cultured pearl, technically
a blister or cavity pearl, is called the Mabé
pearl.
To produce mabé pearls, hollow, flat-
bottomed, plastic domes are inserted in the
space between the mantle and nacreous layer
of the pearl oyster shell (adhered to thenacreous layer). The oyster secretes nacre
on these domes.
In a year or less, the mabés are cut from the
oyster shell and the plastic domes removed.
The hollow interior of each pearl is filled with
wax (sometimes coloured to give the pearl a
slight colour tint) for support, and a disc of
mother of pearl is glued to the bottom.
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Mabé pearls are typically used in pieces of jewellery that
do not necessitate a perfectly spherical shape (e.g.
earrings). Obviously, many different pearl shapes are
possible in this technique through use of variably shaped
plastic “nuclei”.
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Prototypes of Mabé Pearls
Although Mabé pearls are a
relatively recent invention, it is
interesting to note that the same
basic method of blister pearling
bivalves was used by the Chinese
as early as the 5th century A.D.
Carved pieces of ivory, ceramic and
shell were inserted in freshwater
clams to “pearlize” the object.
Elaborate blister pearls are still
being made in China today.
Blister pearl
Buddhas
(5th century)
Modern blister pearl
of Mao
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Nacre:
the natural beauty of mother of pearl
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The Mantle: A Common Characteristic of Molluscs
All molluscs possess:
A fleshy foot, a radula (rasping organ-bivalves have lost this feature), adigestive system, and gills (labelled “ctenidium” here)……but most importantly, for purposes of this lecture, a mantle (a fleshymembrane of tissue that surrounds the visceral mass).
Generic mollusc(showing features common to all molluscs)
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The mantle not only serves to protect delicate internal tissues,but is also responsible for shell secretion (in forms that have ashell). Calcium in molluscan blood reacts with dissolved carbondioxide to result in the precipitation of solid calcium carbonateused in the construction of the various layers of the shell.
The Mantle: The Key to Shell Construction
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At the leading edge of growth,the mantle secretes prisms ofcalcium carbonate (aragoniteor calcite).
The mantle then covers the
prismatic layer with tablets ofaragonite nacre (this is themother of pearl layerobserved on the shellinterior).
Note that when shellsecretion is not taking place,the mantle separates from theshell.
Function of the Mantle
Cross section of pearl oyster shell
Interior of pearl oyster shell
Prismaticlayer
Nacreouslayer
Flaps ofmantletissue
periostracum
(water-filled space)
Prismaticlayer
Nacreouslayer
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On top of the prismatic layer, anorganic material called theperiostracum is deposited (providingprotection from dissolution and
mechanical damage and, to someextent, camouflage).
The drab exterior of the pearl oyster(and other molluscs) conceals thebeauty within. Don’t judge a book by
its cover !
Shell exterior(covered by periostracum)
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Prismatic layer(dull)
Nacreous layer(pearly)
Internal Structure of Shell
The prismatic and nacreous layers have different optical properties due todifferences in crystal habit. The prismatic layer (composed mostly of blockyprisms of calcite or aragonite) tends to be weakly translucent to opaque.
The nacreous layer (composed mostly of plate-like tablets of aragonite), isshiny, translucent and often very colourful.
The smooth fine laminar surface of the nacreous layer allows mantle tissue to
slide against the shell without being damaged.
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A Closer Look at Nacre
Nacre is largely, but not entirely, composed of aragonite crystals; films of
organic matter (specifically as the substance conchiolin) and water arealso present within the nacreous layer.
The general composition of mother of pearl (and pearls) is as follows:
Aragonite (82-86 %)Tablets of aragonite form the framework of nacre
Conchiolin (10-14 %)This is a complex organic substance (C32H48N2O11) made ofpolysaccharides (complex sugars) and protein fibres.
Water (2-4 %)Most of this water occurs in the conchiolin layers.
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Structure of Nacre: Cross sectional view
This is an edgewise (crosssectional--shell cut across itslength or width) view of nacre asobserved under SEM (conchiolinhas been dissolved in this sample)
Tablets of aragonite are glued toadjacent tablets with conchiolin.
Individual tablets can form thicker
sheets, with intervening sheets ofconchiolin.
Sheets of aragonite tablets
held together by conchiolin
Thicker sheets of conchiolinbetween sheets of aragonitetablets
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Structure of Nacre: Plan view
This is an surface (plan) view ofnacre as observed under SEM.
In this image, the hexagonal
shape of the aragonite tablets canbe observed.
Note that the aragonite sheets donot uniformly cover the surface;
they partially overlap one another,forming a step-like pattern.
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Lustre
The quality of lustre in nacre is afunction of two major things:
1. Quality of surface reflection:Aragonite tablets behave asmirrors. The ability of thesurface layer to reflect lightdetermines the brilliance of thelustre.
2. Quality and depth of internalreflection: Aragonite tablets alsobehave like windows – theytransmit some of the incoming
light. Light can be reflected offinternal crystal surfaces, givingnacre a warm internal glow.Generally, the thicker the nacreis, the more reflective (shiny) it
will tend to be as a result.
Surface reflection
Internal reflection
Orient
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Orient
The iridescent play of colours in nacre is called orient
The intensity of orient is dependent on similar factors as those that
produce lustre: the reflection of light off surfaces and the behaviour of lightwithin the nacre (internal reflection, diffraction, dispersion).
Details of these concepts are impossible to explain without the use ofmathematical equations, so we’ll just stick to the basic ideas!
At least you should know (remember) thatvisible light and other EM radiation haswave properties, and therefore, is subject torefraction, diffraction, dispersion andinterference (constructive and destructive).
O i I fl f S f R li f
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Orient: Influence of Surface Relief
One contributor to orient is the splittingof light waves into individual colours of
the spectrum due to the regulararrangement of layered bands ofgrooves and ridges on a surface.
At certain angles of viewing, waves of
certain colours (each reflected at aspecific angle) are reinforced, makingthose colours more brilliant. This iscalled constructive interference.
The same principle applies toiridescence of the surface of a compactdisc which is characterized byalternating lines of pits and ridges(lands). These produce what is knownas a diffraction grating.
grooves and ridges on nacre
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Orient: Influence of Refraction and Reflection
Individual crystals of aragonitecan also act as tiny prisms,refracting light and dispersing it
into the colours of the rainbow.
This effect is further enhancedby the interaction of outgoinglight waves (refraction and
dispersion going in and out) thathave bounced off multiplecrystal surfaces within thesheets of nacre (constructiveinterference).
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Uses of Nacre
Nacre has many applications in rawform.
A popular practice among some shellcollectors is to remove the outerprismatic layer of a shell to reveal the
more attractive nacreous layer.
It is also a popular material for jewelry,inlays in musical instruments, andvarious other ornamental applications.
Nacre has also been widely used formaking buttons.
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Ammolite: Fossil Nacre
A gemstone that has only recentlyentered the market is ammolite.
Ammolite, fossil ammonite nacre, israther rare because under normalpreservational circumstances,aragonite either dissolves or is
recrystallized to the more stable formof calcium carbonate, calcite.
As you will recall, ammonites areextinct relatives of the Nautilus,
squids, octopuses and cuttlefishes.
Like Nautilus, ammonites had achambered shell filled with gas andliquid for buoyancy regulation.
modernNautilus
ammonite
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Orient in Ammolite
Ammonites with exceptionally wellpreserved nacre occur inthe Late Cretaceous BearpawShale, south of Lethbridge Alberta
(about 70 million years old).
For reasons still unanswered, theplay of colours in ammonite nacrefrom the Bearpaw Shale have been
greatly enhanced in intensity due toconstructive interference (this mighthave to do with slight deformationof aragonite crystals within thenacreous layers) or the presence ofimpurities.
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Orient in AmmoliteAmmolite is somewhat difficult towork with because it readily splitsapart along planes between
aragonite sheets (low tenacity)
It is also quite soft and is prone toscratching (low hardness).
The ammolite must therefore beprocessed in a different way thanmost gemstones.
Sheets of ammolite are groundand polished, attached to a
backing (either pieces of theoriginal matrix or harder material),and capped with a cabochon ofquartz or spinel (required toprotect it from scratching or
splitting).
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Thanks For your attention