Natural and Cultivated Pearls
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.
A well equipped gem testing laboratory is able to separate natural pearls from their cultured pearls, non-nucleated cultured pearls and imitation 'pearls' counterparts. The separation between a natural pearl or cultured pearl is almost impossible without a bona-fide gemological X-ray certificate.]Also a well known fact is that pearls in cultivated areas have a different structure then natural pearls.
Pearls come in eight basic shapes: round, semi-round, button, drop, pear, oval, baroque, and ringed. Perfectly round pearls are the rarest and most expensive, and are generally used in necklaces, or strings of pearls. Semi-rounds are also used in necklaces or in pieces where the shape of the pearl can be disguised to look like it is a perfectly round pearl. Button pearls are like a slightly flattened round pearl and can also make a necklace, but are more often used in single pendants or earrings where the back half of the pearl is covered, making it look like a larger, round pearl.
Drop and pear shaped pearls are sometimes referred to as teardrop pearls and are most often seen in earrings, pendants, or as a center pearl in a necklace. Baroque pearls have a different appeal to them than more standard shapes because they are often highly irregular and make unique and interesting shapes. They are also commonly seen in necklaces. Ringed pearls are characterized by concentric ridges, or rings, around the body of the pearl.
In general, cultivated pearls are less valuable than natural pearls, and imitation pearls are the least expensive. One way that jewelers can determine whether a pearl is cultivated or natural is by x-raying the pearl. If the grit in the centre of the pearl is a perfect sphere, then the jeweller knows it is cultivated. This is because when the cultivators insert the grit, (usually a polished piece of mussel shell), it is always perfectly round, so as to produce a more expensive, perfectly round pearl. If the centre is not perfectly round, the jeweller recognizes that it is genuine, and gives it a higher value. Imitation pearls are much easier to identify by jewelers. Some imitation pearls are simply made of mother-of-pearl, coral or conch, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient. Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly.
There is also a unique way of naming pearl necklaces. While most other necklaces are simply referred to by their physical measurement, strings of pearls have their own set of names that characterize the pearls based on where they hang when worn around the neck. A collar will sit directly against the throat and not hang down the neck at all; they are often made up of multiple strands of pearls. Pearl chokers nestle just at the base of the neck. The size called a princess comes down to or just below the collarbone. A matinee of pearls.
Pearls are produced by a variety of organisms, commonly sea mollusks. They are also produced by fresh water mussels, and occasionally, by snails. Some examples of pearl-producing oysters (you don't have to remember these) are:
* Akoya pearl oysters (Pinctada fucata)
* Black Lip Pearl shell (Pinctada margaritifera)
* Freshwater mussel (Hydriopsis schlegeli)
* Large winged pearl shell (Pteria penguin)
* Abalone (Notohaliotis discus)
* Golden Lip pearl shell or white lip pearl shell (Pinctada maxima)
Reference: Mikimoto Pearl Museum, Toba.
Mabe pearls are cultured blister pearls. These are produced by inserting a half bead against the shell of the mollusk, after a layer of nacre has been deposited over the bead, the whole formation is cut out and the nacreous dome cemented onto a mother of pearl bed.
Biwa pearls are produced at lake Biwa, Japan using freshwater clams. They are irregular in shape but have good color and luster. Instead of a bead a small square of mother of pearl in inserted into the clam. These pearls require three years to produce good results.
Concentric layers of CaCO3 are deposited around an irritant. This may be a piece of mantle lobe or some other material.
Only the mantle lobe can secrete nacre. When a piece of mantle lobe is introduced by some accident into the tissue of the oyster, the oyster forms a bag known as a "pearl sac". It is this sac that secretes the nacre around the irritant to make the pearl.
Thus, pearls are calcareous concretions.
Some natural pearls have quite unusual shapes. These are often called "baroque" pearls.
Both saltwater and freshwater pearls consist of the same material and can form in "baroque" shapes. Unless you are quite familiar with the typical characteristics ("look") of pearls from certain specific sources, it would be very difficult to know whether a given pearl was saltwater or freshwater in origin.
Probably the most common freshwater pearl on the market is the Chinese freshwater baroque, some of which are crinkly and look like crisped rice. These have been very popular in recent years because they cost dramatically less than Akoya cultured pearls.
Blister pearls form on the inside of the mother of pearl shell.
Oysters and mussels are induced to make pearls. The result are termed "cultured pearls".
Maybe 90% of the pearls sold are cultured.
If you break a pearl open you will see that it consists of a bead covered by a thin layer of nacre.
The culturing process involves inserting a small piece of mantle lobe and a bead made from mother of pearl shell into the tissues of a pearl-producing mollusk.
The mollusk treats the bead as an irritant and the mantle lobe tissue begins to deposit a nacreous coating over it.