Pearls: irritants, iridescence and industry
Pearl Oysters vs. Edible Oysters
Most people know that precious pearls are made by pearl oysters (which, by the way, are different from the common edible oyster-more closely related to scallops than to true oysters). Edible oysters (true oysters) can also produce pearls, but these are not nacreous (edible oysters do not secrete nacre--calcitic shells produce lustreless calcareous concretions).
Pearl oyster
Edible oyster
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 that secretes a shell is capable of producing a pearl, but highlustre (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 in the 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 by surrounding the irritating body with smooth layers of nacre).
Cross section of natural pearl showing layers of aragonite (separated by layers of conchiolin).
Rarely do grains of sand form the nucleus of a pearl (oysters are quite efficient at expelling sediment particles)
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.
Parasite/intruder
Blister pearl
Blister Pearls Nacreous layer (blue)
Mantle (grey)
Nacreous layer Prismatic layer
In this case, nacre was secreted around a clam that managed to bore into an abalone shell from the outside of the shell.
In this remarkable specimen, a fish somehow got trapped between the mantle and nacreous surface of a pearl oyster. The fish has been covered with nacre, forming a blister.
Free pearls Nacreous layer (blue) Free pearl
Mantle (grey)
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 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 (dark pearls 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)
How rare are natural pearls ? One out of about 10,000-15,000 pearl oysters will produce a natural free pearl Most of these lack the desired spherical shape, but large, irregular pearls (called Baroque pearls) have commanded high prices throughout history.
Note the term “Baroque� (originally from the Portuguese term barroco, meaning unpredictable or elaborate), was used to describe pearls long before it gained meaning in a art or music.
Baroque pearl set in gold
Fossil pearls As the nacreous layer of shells can sometimes be preserved in the fossil record, so too can pearls (although these are extremely rare). These are fossil pearls of pen shells from Eocene (50 million years old) London Clay – they retain their nacreous lustre 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 shape 3. 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 by other 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.
Shells of freshwater mussels are cut and polished to make nucleation beads for cultured pearls.
A nucleation bead and a strip of donor tissue are inserted in the gonad of the pearl oyster
A pearl sac forms in the gonad. The epithelial mantle tissue continues to secrete nacre and, if all goes well, covers the bead with nacre to form a free pearl. Natural pearls generally have a large 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).
Large nucleus (nucleation bead)
Small nucleus
Natural pearl
Cultured pearl
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 a Japanese pearl farm: 500 die during the culturing period 250 produce poor-quality pearls 200 produce saleable pearls of low to medium quality 50 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) China Japan 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, flatbottomed, plastic domes are inserted in the space between the mantle and nacreous layer of the pearl oyster shell (adhered to the nacreous 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.
Blister pearl Buddhas (5th century)
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.
Modern blister pearl of Mao
Thanks for you attention
Nacre: the natural beauty of mother of pearl
Mother Of Pearl Mother of pearl is a common term for lustrous, iridescent material forming the inner surface of (molluscan) seashells.
gastropods Turban shell
Abalone
The material comprising mother of pearl is called nacre. Nacre production is widespread among molluscs, the invertebrate group (Phylum) that includes the bivalves (clams, mussels and oysters), the gastropods (snails) and cephalopods (primarily Nautilus and extinct ammonites)
bivalves
Pearl oyster
Freshwater clam
cephalopod
Nautilus
The Mantle: A Common Characteristic of Molluscs All molluscs possess: A fleshy foot, a radula (rasping organ-bivalves have lost this feature), a digestive system, and gills (labelled “ctenidium” here)… …but most importantly, for purposes of this lecture, a mantle (a fleshy membrane of tissue that surrounds the visceral mass).
Generic mollusc (showing features common to all molluscs)
The Mantle: The Key to Shell Construction The mantle not only serves to protect delicate internal tissues, but is also responsible for shell secretion (in forms that have a shell). Calcium in molluscan blood reacts with dissolved carbon dioxide to result in the precipitation of solid calcium carbonate used in the construction of the various layers of the shell.
Function of the Mantle Prismatic At the leading edge of growth, layer the mantle secretes prisms of Nacreous calcium carbonate (aragonite layer
periostracum
(water-filled space)
or calcite). The mantle then covers the prismatic layer with tablets of aragonite nacre (this is the mother of pearl layer observed on the shell interior). Note that when shell secretion is not taking place, the mantle separates from the shell.
Cross section of pearl oyster shell Prismatic layer Nacreous layer Flaps of mantle tissue
Interior of pearl oyster shell
On top of the prismatic layer, an organic material called the periostracum is deposited (providing protection from dissolution and mechanical damage and, to some extent, camouflage). The drab exterior of the pearl oyster (and other molluscs) conceals the beauty within. Don’t judge a book by its cover !
Shell exterior (covered by periostracum)
Internal Structure of Shell Prismatic layer (dull)
Nacreous layer (pearly) The prismatic and nacreous layers have different optical properties due to differences in crystal habit. The prismatic layer (composed mostly of blocky prisms of calcite or aragonite) tends to be weakly translucent to opaque. The nacreous layer (composed mostly of plate-like tablets of aragonite), is shiny, 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 are also 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 of polysaccharides (complex sugars) and protein fibres. Water (2-4 %) Most of this water occurs in the conchiolin layers.
Structure of Nacre: Cross sectional view Sheets of aragonite tablets held together by conchiolin
This is an edgewise (cross sectional--shell cut across its length or width) view of nacre as observed under SEM (conchiolin has been dissolved in this sample) Tablets of aragonite are glued to adjacent tablets with conchiolin. Individual tablets can form thicker sheets, with intervening sheets of conchiolin.
Thicker sheets of conchiolin between sheets of aragonite tablets
Structure of Nacre: Plan view
This is an surface (plan) view of nacre as observed under SEM. In this image, the hexagonal shape of the aragonite tablets can be observed. Note that the aragonite sheets do not uniformly cover the surface; they partially overlap one another, forming a step-like pattern.
Lustre The quality of lustre in nacre is a function of two major things: 1. Quality of surface reflection: Aragonite tablets behave as mirrors. The ability of the surface layer to reflect light determines the brilliance of the lustre. 2. Quality and depth of internal reflection: Aragonite tablets also behave like windows – they transmit some of the incoming light. Light can be reflected off internal crystal surfaces, giving nacre a warm internal glow. Generally, the thicker the nacre is, the more reflective (shiny) it will tend to be as a result.
Surface reflection
Internal reflection
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 light within the nacre (internal reflection, diffraction, dispersion). Details of these concepts are impossible to explain without the use of mathematical equations, so we’ll just stick to the basic ideas!
At least you should know (remember) that visible light and other EM radiation has wave properties, and therefore, is subject to refraction, diffraction, dispersion and interference (constructive and destructive).
Orient: Influence of Surface Relief One contributor to orient is the splitting of light waves into individual colours of the spectrum due to the regular arrangement of layered bands of grooves and ridges on a surface. At certain angles of viewing, waves of certain colours (each reflected at a specific angle) are reinforced, making those colours more brilliant. This is called constructive interference. The same principle applies to iridescence of the surface of a compact disc which is characterized by alternating lines of pits and ridges (lands). These produce what is known as a diffraction grating.
grooves and ridges on nacre
Orient: Influence of Refraction and Reflection
Individual crystals of aragonite can also act as tiny prisms, refracting light and dispersing it into the colours of the rainbow. This effect is further enhanced by the interaction of outgoing light waves (refraction and dispersion going in and out) that have bounced off multiple crystal surfaces within the sheets of nacre (constructive interference).
Uses of Nacre Nacre has many applications in raw form. A popular practice among some shell collectors is to remove the outer prismatic layer of a shell to reveal the more attractive nacreous layer. It is also a popular material for jewelry, inlays in musical instruments, and various other ornamental applications. Nacre has also been widely used for making buttons.
Ammolite: Fossil Nacre A gemstone that has only recently entered the market is ammolite.
modern Nautilus
Ammolite, fossil ammonite nacre, is rather rare because under normal preservational circumstances, aragonite either dissolves or is recrystallized to the more stable form of calcium carbonate, calcite. As you will recall, ammonites are extinct relatives of the Nautilus, squids, octopuses and cuttlefishes. Like Nautilus, ammonites had a chambered shell filled with gas and liquid for buoyancy regulation.
ammonite
Orient in Ammolite
Ammonites with exceptionally well preserved nacre occur in the Late Cretaceous Bearpaw Shale, south of Lethbridge Alberta (about 70 million years old). For reasons still unanswered, the play of colours in ammonite nacre from the Bearpaw Shale have been greatly enhanced in intensity due to constructive interference (this might have to do with slight deformation of aragonite crystals within the nacreous layers) or the presence of impurities.
Ammolite is somewhat difficult to work with because it readily splits apart along planes between aragonite sheets (low tenacity) It is also quite soft and is prone to scratching (low hardness). The ammolite must therefore be processed in a different way than most gemstones. Sheets of ammolite are ground and polished, attached to a backing (either pieces of the original matrix or harder material), and capped with a cabochon of quartz or spinel (required to protect it from scratching or splitting).
Orient in Ammolite
Thanks For your attention