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“All art is autobiographical; the pearl is the oyster’s autobiography,” said Italian film-maker Frederico Fellini, observing no two pearls are precisely similar. Yet all oysters grow stunningly symmetrical, perfectly round pearls, which has remained an enigma for centuries.

The work of Mexican artist Jorge Méndez Blake is illuminating in understanding the principle of paracrystallinity. All the bricks have to be the same size; if not, the wall would be uneven. With uneven walls, it is not at all possible to build skyscrapers.

A single book placed below the brick layer causes a flaw which propagates all along the wall

He placed a single book under the brick wall, showing how this single defect traverses the bricklayers. One defective site propagates throughout the whole body. The flaw in the staking order of bricks will weaken the built structure, which is why the masons use external templates, such as bubble level or spirit level,  guideposts and so on, to ensure local disorders do not propagate throughout the structure.

The oyster also builds the pearl layer with nacre, just as the mason builds the tall buildings stacking bricks. While secreting nacre, here and there, it is normal for minor defects to creep in. However, amazingly the ultimate shape of pearls is flawless. While the oversight by the mason in case of construction removes the stains, what drives the elimination of flaws in the case of the pearls?

Led by Australian National University’s biochemist, Laura Otter, a research team has unravelled the fascinating mechanism behind the mystery of flawless, symmetrical growth of pearls.

Dr Laura Otter

How pearls are produced

Pearls grow within the soft tissue of oysters. While all molluscs produce some kind of pearl, not all of them are valued as gemstones. Only pearls produced by molluscan bivalves or clams are valued for the solid and shining property of the nacre secretion.

A foreign body irritates the oyster and to heal it the oyster secrets nacre, which grow to become a pearl.

A pebble that got stranded in the sole of the shoe irritates; fishbone stuck in throat stings. Likewise, when a grain of sand or other impurities somehow makes its way into the interior, it annoys the oyster.  Riled by the irritant, the oyster begins coating it in a material it secrets called nacre. First, the object is sheathed with the epithelium layer. Layer by layer, the nacre is coated around it, eventually forming a pearl. The pearls grow attached to the inside of the shells.

Aragonite bricks are bound by conchiolin mortar to make a layer of strong nacre

A diamond is nothing but one form of carbon with special atomic arrangements; likewise, 80% of pearl is nothing but aragonite, a kind of calcium carbonate in the form of CaCO3. About 10-14% is conchiolin, one type of organic material. 2%–4% is water. In addition, depending upon the nature of the organism oozing the nacre, certain special classes of proteins are also part of the composition. These proteins provide the unique characteristics of the pearls.

The lustre of pearls

The pearl is the queen of gems and the gem of queens. All other gemstones are formed in the geological process involving high pressure and heat. The pearl is a product of a living being. All the gemstones have to be polished to unveil their magnificence; however, pearls are the only gemstone that need not be polished to bring out their splendour.

As each layer reflects part of the light incident on it, the pearls appear to shine; metals like sodium give it various shades of colour.

Light passing through each layer of the nacre is partially reflected and partially absorbed, giving the irradiance of the pearl. A lightray penetrating inside the pearl is reflected back by one of the thin layers of nacre. As different layers reflect back, the pearls acquire their characteristic lustre: white, cream, yellow, red, green, blue and even black. When all the incident light is absorbed, the pearl appears black; and the black ones are a rarity having high market value.

Pearls come in various shapes, but the round ones are considered precious

The pearls take the shape of sphere, teardrop or oval. At times the pearl can become half sphere, like a button. However, the perfectly spherical ones are highly valued. Peals that are bigger in size and flawlessly round are the most costly ones.

Investigation

“Pearls don’t lie on the seashore, if you want one, you must dive for it,” says Chinese Proverb. Lara Otter and her team examined the structure of the layers inside the pearl to understand the mechanism being the mystery.

An irregularly shaped grain is the seed for the pearls to grow. As layers grow around it, the asymmetrical shape of the seed will induce flaws carried across the layers. Yet, it is truly amazing that the pearl grows to be perfectly symmetrical.

Lara Otter and her team examined the structure of the layers inside the pearl to understand the mechanism being the mystery. The researchers used Keshi pearls collected from Akoya pearl oysters (Pinctada imbricata fucata). The pearls were about 50 mm in size. Using diamond wire, the researchers cut sample pieces the size of 3 to 5 mm. Using the electron microscope, the researchers peered to look at structures as small as 3-nanometre size. They found  2,615 layers of nacre in a test pearl, and the pearl took  548 days to produce this mature pearl.

Deformities

Only with great calculated effort can we build skyscrapers and ensure the same number of aligned bricklayers on all sides. If the brick arrangements had a flaw, cracks would appear in the building, and perhaps it may even collapse. The oyster also builds the pearls by adding 500 nanometre thick layers, one above the other.

Initial layers show deformities and imperfections generated by seed shape, however in later stages the pattern of growth ensures to smoothen the irregularities making the ultimate product, pearl, symmetrical

The irritant that kickstarts the oozing of nacre and the growth of the pearl is irregular in shape. Naturally, the layer of nacre forming around this would be jagged. The researchers found that the first few layers of the nacre are indeed not smooth. Further, they also noticed that the nacre layers were not even, but some were thin and some slightly thick. Due to the difference in the size of the layers, researchers could see the disorder creeping in. Nevertheless, the average thickness of hundreds of layers at all points was the same, erasing the defects.

Mechanism of repair

While making boondhi laddu, we take a small portion in our hand, press down with our palm and rotate it to give it a spherical shape. Likewise, once in 20 days, the grown pearl turns, says a 2005 study. Therefore, any differences in growth rate along the axis of rotation get copied around the entire circumference giving the pearl its rotational symmetry.

In their research Otter and the team noticed a pattern in the formation of layers. If a particular layer was thick, the next layer, the researchers noticed, was thin and vice versa.  When the thick and thin alternate, the average thickness of the coating becomes constant, irrespective of the differences in the thickness of the individual layers.

They also noticed that any surface defect that is randomly produced is erased within 16 layers or so with this mechanism. That is, meso-level structural order is achieved in around 20 layers. However, the order produced over longer distance had to do with what is called pink noise, the researchers found.

Pink noise

Also called Flicker noise, pink noise is intrinsic to electronic devices such as resistors and amplifiers. When you roll the dice, the number we get in the first roll has no connection whatsoever with the second roll. The first result has no connection with the second. The numbers are random. However, in many natural phenomena, there is a coupling between many factors. The earlier happening could have an impact on future outcomes, even though the whole system is indeterministic. In some of the cases, the relationship follows what is known as 1/f noise pattern. In 1/f noise or pink noise, the frequencies are usually lower and second, the power of the wave is inversely proportional to the frequency. In the case of the pearl growth, f=1/(number of layers)

The thickness of the layers initially was random, however in the mid level thick and thin alternated. In the mature phase the layers followed the 1/f pink noise pattern

When a multi-storied building is built, in a sense, each brick has its position predetermined. However, in the case of pearls, neither the size of the thickness nor the place of each layer is prearranged in the nacre layer growth. Nevertheless, within 20 layers, some sort of order emerges from the chaos. When one took hundreds of layers, the 1/f function helped understand the long-range ordering that appeared in the formation of the pearls.

Pearls in peril

Most of the pearls sold in the market today are cultured and not naturally formed ones. Natural pearls that come from water bodies and seas are scarce; highly-priced. The Indian ocean, East Asia and the Pacific region are studded with pearl culture farms where most of the pearls that we see in jewellery shops are produced. Tiny spherical beads are made from shells, and this is ingested into the oyster. The irritant provides the oyster to secrete nacre enabling the maturation of the pearl.

While the natural pearls have many layers of nacre, but the cultured has only a small outer layer

Climate change is emerging as a threat to the pearls. With oceans warming, the oysters migrate. However, in the new place, it would have to encounter organisms that are already there. More warm water may impede the normal function of the oyster from heat stress. With global oceans warming, more carbon dioxide is absorbed in the warm seawater resulting in acidification of seas. With the water more acid, the oyster shells would be eroded at a much faster rate. Scientists say the oyster would then divert significant nacre resources to fix the shells rather than build the pearls.

(Mathisport is the inmathi.com science column. It is a tribute to the Mindsport column of Mukul Sharma)

TV Venkateswaran is a scientist at Vigyan Prasar, Department of Science and Technology, New Delhi


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