Pearls Introduction

8/9/2019 Pearls Introduction

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Pearls:irritants, iridescence and industry





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)



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.



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.



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.



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



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



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



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)





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



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



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.



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.



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



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.



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



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.



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”.



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



Thanks for you attention



Nacre:

the natural beauty of mother of pearl





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)



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



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



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)



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.



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.



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



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.



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



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



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



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).



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.



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



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.



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).



Thanks For your attention

Pearls Introduction

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