Pearls fit into two categories: freshwater and saltwater. As their name implies, freshwater pearls are formed in freshwater mussels that live in lakes, rivers, ponds and other bodies of fresh water. Most freshwater cultured pearls sold today come from China. By contrast, saltwater pearls grow in oysters that live in the ocean, usually in protected lagoons. Akoya, South Sea and Tahitian are the three main types of saltwater pearls.
Akoya Pearls
The
difference between natural and cultured pearls focuses on whether the
pearl was created by nature, without human intervention, or with human
aid. Pearls are formed inside the shell of certain bivalve mollusks.
As a response to an irritant inside its shell the mollusk creates a
pearl as a defense mechanism. The mollusk will deposit layers of calcium
carbonate (CaCO3) in the form of the minerals
aragonite or calcite (both crystalline forms of calcium carbonate) held
together by an organic horn-like compound called conchiolin. This combination
of calcium carbonate and conchiolin is called nacre, or as most know
it, mother-of-pearl. The commonly held belief that a grain of sand acts
as the irritant is in fact rarely the case.
Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the animal's body. These small particles or organisms enter the animal when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically a cut piece of the mantle epithelium, together with processed shell beads, the combination of which the animal accepts into its body.
Natural pearls are nearly 100% nacre. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters a bivalve mollusk, and settles inside the shell. The mollusk, being irritated by the intruder, secretes the calcium carbonate substance called nacre to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with round ones being comparatively rare.
'Cultured' pearls (nucleated and non-nucleated or tissue nucleated cultured pearls) and imitation pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often 'pre-formed' as they tend to follow the shape of the implanted shell bead nucleus. Once the pre-formed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more. When a nucleated cultured pearl is X-rayed it will reveal a different structure to that of a natural pearl. It exhibits a solid center with no concentric growth rings, compared to a solid center with growth rings.
A well equipped gem testing laboratory is able to separate natural pearls from cultured pearls by examining the center of a pearl, and the growth rings separated by conchiolin layers. The differentiation of a natural pearl or tissue-nucleated cultured pearl can be difficult without a gemological X-ray.
All natural and cultured pearls can be distinguished from imitation pearls by microscope. Another accurate method of testing for imitations is rubbing the pearl against the biting edge of a front tooth. While imitation pearls feel completely smooth, natural and cultured pearls are composed of nacre platelets, which feel slightly gritty.
Quality natural pearls are very rare jewels. Actual value of a natural pearl is determined as other "precious" gems. Valuation factors include size, shape and quality of surface, orient, and luster.
Single natural pearls are often sold as a collector item, or set as centerpieces in unique jewelry. Very few matched strands of natural pearls exist, and those that do often sell for hundreds of thousands of dollars. Yachtsman and financier Cartier purchased the landmark Cartier store on Fifth Avenue in New York for $100 cash and a double strand of matched natural pearls valued at $1 million.
Keshi pearls, although they often occur by chance, are not considered natural pearls. They are a byproduct of the culturing process, and hence do not happen without human intervention. These pearls are quite small: typically a few millimeters in size. Keshi pearls are produced by many different types of marine mollusks and freshwater mussels in China. Today many "keshi" pearls are actually intentional, with post-harvest shells returned to the water to regenerate a pearl in the existing pearl sac.
Previously natural pearls were found in many parts of the world. Present day natural pearling is confined mostly to seas off Bahrain. Australia also has one of the world's last remaining fleets of pearl diving ships. Australian pearl divers dive for south sea pearl oysters to be used in the cultured south sea pearl industry. The catch of pearl oysters is similar to the numbers of oysters taken during the natural pearl days. Hence significant numbers of natural pearls are still found in the Australian Indian Ocean waters from wild oysters. X-Ray examination is required to positively verify natural pearls found today.
Black
pearls, frequently referred to as Black Tahitian Pearls, are highly
valued because of their rarity; the culturing process for them dictates
a smaller volume output and can never be mass produced. This is due
to bad health and/or non-survival of the process, rejection of the nucleus
(the small object such as a tiny fish, grain of sand or crab that slips
naturally inside an oyster's shell or inserted by a human), and their
sensitivity to changing climatic and ocean conditions. Before the days
of cultured pearls, black pearls were rare and highly valued for the
simple reason that white pearl oysters rarely produced natural black
pearls, and black pearl oysters rarely produced any natural pearls at
all. Since pearl culture technology, the black pearl oyster found in
Tahiti and many other Pacific Island area has been extensively used
for producing cultured pearls. The rarity of the black cultured pearl
is now a "comparative" issue. The black cultured pearl is
rare when compared to Chinese freshwater cultured pearls, and Japanese
and Chinese Akoya cultured pearls, and is more valuable than these pearls.
However, it is more abundant than the south sea pearl, which is more
valuable than the black cultured pearl. This is simply due to the fact
that the black pearl oyster Pinctada margaritifera is far more abundant
than the elusive, rare, and larger south sea pearl oyster - Pinctada
maxima, which cannot be found in lagoons, but which must be dived for
in a rare number of deep ocean habitats. Black cultured pearls from
the black pearl oyster — Pinctada margaritifera — are NOT
south sea pearls, although they are often mistakenly described as black
south sea pearls. In the absence of an official definition for the pearl
from the black oyster, these pearls are usually referred to as "black
Tahitian pearls". The correct definition of a south sea pearl —
as described by CIBJO and the GIA — is a pearl produced by the
Pinctada maxima pearl oyster. South sea pearls are the color of their
host Pinctada maxima oyster — and can be white, silver, pink,
gold, cream, and any combination of these basic colors, including overtones
of the various colors of the rainbow displayed in the pearl nacre of
the oyster body itself.
The
largest pearl ever found came from the Philippines in 1934. Although
referred to as a pearl, it is actually a non-nacreous calcareous concretion
- gemologically speaking, it is not actually a pearl. It weighs 14 lb
(6.4 kg) and was discovered by an anonymous Filipino Muslim diver off
the island of Palawan in 1934. Later, a Palawan chieftain gave the pearl
to Wilbur Dowell Cobb in 1936 as a gift for having saved the life of
his son. The pearl had been named the Pearl of Allah
by the Muslim tribal chief because it resembled a turbaned head. In
1980, Cobb's heirs sold it to a jeweler in Beverly Hills, California,
for $200,000. It is now estimated to be worth upwards of $40,000,000!
Currently, the Philippines produces both white and black pearls 8 to
20mm. However, they are most known for colors ranging from silvery white
to champagne gold. The south sea pearl is the national gem of the Philippines.
A legend purports that the Pearl of Allah is actually the Pearl of Lao-Tzu, a cultured pearl created with a carved amulet and progressively grafted into several giant clams, but was lost due to a shipwreck in 1745, only to be rediscovered in 1934. The legend has been discredited, however. The pearl is the product of a Tridacna gigas, which cannot be grafted. The pearl is also a whole pearl, not a mabe. Whole pearl culturing technology is only 100 years old.
The value of the pearls in jewelry is determined by a combination of the luster, color, size, lack of surface flaw and symmetry that are appropriate for the type of pearl under consideration. Among those attributes, luster is the most important differentiator of pearl quality according to jewelers. All factors being equal, however, the larger the pearl the more valuable it is. Large, perfectly round pearls are rare and highly valued.
Pearls come in eight basic shapes:
