Factors that determine the shape of pearls

Pearls occur in seven basic shapes in nature. These are 1) round 2) near-round 3) oval 4) button 5) drop 6) semi-baroque and 7) baroque. The GIA classifies pearl shapes into three main categories. They are :- 1) Spherical 2) Symmetrical and 3) Baroque.
1) Spherical - Spherical pearls have a uniform or near-uniform diameter all round and several lines of symmetry through which the pearl can be divided into two equal halves. Round and near-round pearls come under this category.
2) Symmetrical - Symmetrical shapes have only a single median line of symmetry through which the pearl can be divided into two equal halves. Oval, button and drop shapes come under this category of pearl shapes.
3) Baroque - Baroque pearls have an irregular shape without a line of symmetry. Semi-baroque and baroque pearls fall under this category.

To what extent do these different shapes occur in nature ?

The commonest shape found in natural pearls, be they saltwater or freshwater is the irregular or baroque shape, followed by the rare symmetrical shapes, and the rarest of all shapes the spherical or perfectly round shapes. This fact is clearly brought out in our table of famous nacreous and non-nacreous single pearls arranged in descending order of weights, as appearing in our webpage on the famous Arco-Valley Pearl.. According to this table, the first six largest natural pearls in the world are all baroque pearls. These pearls are the Pearl of Allah, Palawan Princess, Pearl of Asia, Arco-Valley Pearl, Big Pink Pearl and the Hope Pearl. The seventh is a rare spherical melo pearl,the Bao dai/Sunrise pearl, but more baroque pearls follow after this,such as the 8th, 9th, 12th, 14th and 31st pearls. The next most commonest are the symmetrical pearls, under which come the pear-shaped drop, the tear-drop shape, oval and button-shaped pearls, such as pearl 10,11, 13, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 32, 33,34 & 35. The least common are the spherical and near-spherical shapes, such as 7, 16, 27, 28, 29, 30,36 & 37.

What are the largest pearls in each of the categories you have mentioned ?

The largest baroque pearl is without any doubt the non-nacreous Pearl of Allah, weighing 6 Kg or 30,000 carats or 120,000 grains. The largest nacreous baroque pearl is the 600-carat or 2,400-grain Pearl of Asia. The largest Abalone baroque pearl, is the 470-carat (1,880-grain) Big Pink Pearl. However, a natural abalone baroque pearl weighing 685 carats, discovered off the coast of California in 1996, and presently in the personal collection of K.C. Bell, appears to have overtaken the Big Pink Pearl as the largest abalone pearl in the world.

The following thumbnails are :- 1) Pearl of Allah and 2) Pearl of Asia - two different views of the pearl in the bunch-of-fruits setting.
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Under the symmetrical pearl category the largest pear-shaped drop-pearl is the 126-carat (504 grains) Gogibus Pearl. The largest symmetrical drop-shaped pearl is the 56.81-carat (227.24-grain) natural brownish-grey pearl, that appeared at a Christie’s auction in Dubai, on April 29, 2008. The largest conch pearls are the symmetrical oval shaped 22.4-carat (89.6-grain) Susan Hendrickson conch pearl. The largest Quahog pearl is the 13.5-carat (54 grains) button-shaped Pearl of Venus.
The following thumbnails are :-
1)Gogibus Pearl - Long lost and re-discovered
2)227.24-grain natural brownish-grey pearl, largest symmetrical drop-shaped pearl in the world.
3)89.6-grain Susan Hendrickson's Conch Pearl - the larger of the two pearls
4)54-grain Pearl of Venus - Largest Quahog pearl

Under the spherical and near-spherical category, the largest perfectly round spherical pearl in the world, is the melo pearl, Bao Dai/Sunrise Pearl. The largest nacreous near-spherical pearl in the world, is the 60.36-carat (241.44-grain) natural freshwater pearl with a combination of yellow to pinkish-orange colors that appeared at a Christie’s Dubai Sale on April 29, 2008.The largest nacreous natural saltwater spherical pearls are the 113.75-grain and 113.25-grain Bapst pearls, reputed to be to be the most extraordinary pair of pearls ever seen in Europe, during the Great Paris Exposition of 1878, where they attracted universal attention.
The following thumbnails are :-
1)Bao Dai/Sunrise Pearl -the largest spherical pearl in the world.
2)241.44-grain natural freshwater pearl - largest nacreous near-spherical pearl in the world.

According to your information the occurrence of natural saltwater nacreous spherical pearls appear to be quite rare.

Yes indeed ! Peter. If information on all existing famous single pearls both of historic as well as modern origin, have been taken into account in our table of famous nacreous and non-nacreous single pearls arranged in descending order of weights, the largest natural, saltwater, nacreous, spherical pearls are the 113.75-grain and 113.25-grain Bapst pearls, of 19th-century provenance that became famous during the Great Paris Exposition of 1878. The rarity of such natural saltwater nacreous spherical pearls is clearly brought out by this fact. It appears that during the 134 years that has lapsed since the first appearance of the spherical Bapst pearls at the Great Paris Exposition of 1878, no other significantly large natural saltwater nacreous spherical pearl had appeared to challenge the position occupied by the Bapst pearls. It is perhaps due to this scarcity and rarity, that the 243.76-grain, cultured, white, saltwater, nacreous, spherical South Sea pearl, known as the Paspaley Pearl received international acceptance as one of the famous pearls of the world.

What are the factors that determine the shape of natural and cultured pearls ?

The main factors that determine the shape of natural and cultured pearls are :-
1) The shape of the irritant for natural pearls,and the implanted nucleus/bead for cultured pearls.
2) The nature of the tissues surrounding the growing pearl and positioning of the implanted nucleus/bead for cultured pearls.
3) Regular turning of seeded oysters for cultured pearls, ensures a uniform deposition of nacre all round resulting in a spherical pearl. Under natural conditions such turning does not take place, resulting invariably in an irregular shaped baroque pearl.
Considering the above factors in detail :-
1)The growing pearl normally assumes the same shape of the irritant for natural pearls or the nucleus/bead for cultured pearls. For natural pearls this fact is clearly borne out by the following examples :-
a) The Survival Pearl - A 90.35-carat natural freshwater baroque pearl in the shape and form of a snail, originating inside a freshwater mussel species in the Tennessee river, is believed to have been formed after the mussel was invaded by a snail-parasite. The reaction of the mussel was to inactivate the parasitic snail and render it harmless, by laying down layer after layer of nacre in the form of alternating layers of conchiolin and aragonite around the snail, a natural response of the freshwater mussel for its survival. Hence, the name Survival Pearl.
b) The Southern Cross Pearls - the renowned cluster of nine pearls in the form of a cross, discovered in Western Australia during the period 1874-1883, from the oyster species Pinctada maxima (Silver-lipped pearl oyster) is believed to have been built around the frond of a serrated seaweed that gained access into the oyster, and served as the irritant.
For cultured pearls the nucleus/bead used for implantation in the gonads,are perfectly round mother-of-pearl beads obtained from a freshwater Mississippi mussel species. Hence, the pearl growing around this bead also assumes the spherical shape of the bead, whose formation is further assisted by regular turning around of the seeded oysters. However, the best example to show that cultured pearls assume the shape of the implanted bead, comes from history; the so called Buddha pearls, produced by ancient Chinese inside the oyster species Pinctada maxima, by implanting tiny beads in the shape of the Buddha in the sitting posture.
The thumbnails shown below are :-
1)The Survival Pearl
2)The Southern Cross Pearl

2) The second important factor that determines the shape of pearls is the nature of the tissues surrounding the growing pearl and positioning of the implanted nucleus/bead for cultured pearls. For natural pearls if the irritant lodges itself in softer tissues deep inside the oyster or mussel, such as the gonads, the growing pearl expands equally on all sides, resulting in a spherical pearl. If on the other hand one or more sides of the growing pearl had tough tissues or the shell itself, as in blister pearls where the irritant lodges between the mantle and the shell, growth on that side will be severely restricted resulting in a baroque-shaped blister pearl, flattened on one side.eg. the Arco-Valley Pearl, Palawan Princess, Pearl of Allah etc. In the case of Melo pearls, large near-spherical and spherical pearls are quite common unlike in other pearls. The larger size and spherical shape are clearly associated with the enormous size of the Melo melo sea-snails. The unusually large size of the sea-snail enables the growth of larger pearls inside the viscera of the snail. The soft tissues in the visceral mass enable equal expansion of the pearl on all sides during its growth, resulting in a large spherical pearl. However, it is not known exactly what tissues in the visceral mass are involved in pearl formation.
In the case of cultured pearls, spherical shape mother-of-pearl beads obtained from the Mississippi freshwater mussel are implanted in the gonads of the oysters, a soft tissue where the cultured pearl grows, expanding equally on all sides, resulting in spherical pearls - eg. cultured South Sea pearls and cultured Tahitian black pearls.
3) Regular turning of the seeded oysters also ensures a smooth and even deposition of nacre on all sides, an important cultural practice for cultured pearls. Under natural conditions such turning does not take place, another reason why natural spherical pearls are extremely rare.
The thumbnails shown below are :-
1) The Arco-Valley pearl - A blister pearl.
2) Spherical melo pearl
3) A South Sea cultured pearl necklace from Princess Margaret's collection
4) Birk's black Tahitian double-row cultured pearl necklace

Out of the three factors mentioned, which in your opinion has the greatest bearing on the shape of pearls ?

The answer to this question depends on whether the pearls are natural or cultured. For natural pearls the factor that has the greatest bearing is the nature of the tissues surrounding the growing pearl. If the irritant lodges in the mantle or between the mantle and the shell, as it often happens in oysters and mussels, the pearls formed are invariably irregular baroque pearls. On very rare occasions when the irritant lodges deep inside the visceral mass such as the gonads, the pearls are spherical or near-spherical. This fact is clearly brought out by the Melo pearls, in which the tendency is to form more spherical pearls, which is clearly associated with the larger size of the Melo sea-snail, with its large and soft visceral mass where the irritant lodges.
For cultured pearls, where the production of the preferred spherical pearl is usually the main target, both factors such as the shape of the implanted nucleus/bead and the positioning of such implanted nucleus/bead have an equal bearing on the shape of the pearl and are taken care of by the Pearl Culturists. For cultured South Sea pearls and cultured Black Tahitian pearls, spherical mother-of-pearl beads obtained from from the Mississippi freshwater mussel are implanted in the gonads of the oysters, a soft tissue deep inside the oysters.