Pearl

Pearl is one of June's birthstones. A classic pearl necklace and/or pearl earrings are one of the most loved gifts one can give. People seem to radiate when wearing pearl jewelry.
Pearl -- The Heart of Purity and Wisdom by Margaret Burgon Klemp
Nathaniel Hawthorne in his epic story, The Scarlet Letter called the innocent little girl portrayed in the story, Pearl. It was a sign of purity and virtue, and he exemplified those traits in the character. The members of the Mormon Church revere one of their texts: The Pearl of Great Price---as one source of the words of wisdom that they live by. The name Margaret and variations of it have connections with the Pearl.
Most people equate precious stones with open pit mines or ancient river beds or some other type of mining. Usually precious stones are found as a result of ancient lava flows and millions of years weathering processes. However, there are other habitats that produce some of the world's finest gems. They spawn in the oceans and in some cases freshwater streams and lakes all over the planet. Pearls, coral and jet are organic gems. The pearl has been found all the way from Asia to some of the rivers and water sources in the United States.
It was the Native Americans living along the eastern seaboard of the United States along with tribes around the Mississippi River Basin who were the first Americans to start collecting pearls. Pearls are unique in that they can be found in both freshwater and saltwater. The stones were highly prized by the Native Americans who used them for decorative purposes, while some tribes used the pearl as a form of currency. It is reported that the father of Pocahontas received pearls as tribute, and it is said that he had a tendency to hoard them.
Since pearls are found in mollusks that can be found in both saltwater and freshwater habitats they are quite plentiful. Pearl production started in the United States in New Jersey around 1850, although there were other smaller enterprises in operation prior to that time such as the Native American enterprises.
The pearl is produced by mollusks which have a nacreous lining. This lining is better known as "mother of pearl." Mollusks are invertebrate animals with soft, unsegmented bodies, such as clams, and snails, usually enclosed in a calcium shell. They are any of numerous chiefly marine invertebrates of the phylum Mollusca. Mollusks can occur in habitats ranging from the deep sea to high mountains. There are eight main classes of mollusks: Gastropoda (snails), Bivalvia (clams, oysters and scallops), Pelecypoda (fossilized bivalves), Cephalopoda (octopus and squid), Scaphopoda (tusk shells), Aplacophora (no shell invertebrates), Caudofoveata (scaly with calcareous spines), Polyplacophora (chitons), and Monoplacophora (ancient fossil shells). Some mollusks can also be utilized as a food source, as well a source that is used in jewelry and decorative items
John Latendresse is known as the founder of the cultured pearl industry in the United States, and it was he who organized the first experimental freshwater pearl farm in Tennessee where experimentation and research would contribute to the whole cultured pearl industry. Since then other farms have started, and the industry has grown.
Pearl is actually made up of layers and layers of Mother of Pearl which gives it an onion like appearance. Aragonite microcrystals are meshed together by horny substance known as Conchiolin which is a protein substance that is the organic basis of mollusk shells. The crystals that manifest in the production of the pearl overlap and results in a roughness that authenticates it. One way it is actually identified is by rubbing it across the teeth. To be an authentic pearl the surface has to be irregular. It if doesn't have a rough surface then the pearl is not genuine.
There is a wide color spectrum among pearls. The general color of a pearl is also called the body color. The color is produced by light interference known as the color "orient" and can be semitransparent to opaque. Orient refers to colors that seem to move around as the gem is turned. Pearls can be white, silver, cream-colored, gold, green, blue, or even black. The color is synonymous with the type oyster or mollusk from which it originated. Various types of oysters will sometimes produce different hues, and water and implants will also make a significant difference in the color. Some pearls have overtones and some don't. An overtone is a color that lays over the body color, and sometimes adds a depth to the piece that might not otherwise be there.
There are two types of pearls available on the gem market: Natural pearls and cultured pearls.
Natural Pearl - A natural pearl is one that forms in nature with no human intervention.
Natural pearls are nearly 100% Mother of Pearl (nacre). Pearls formed under natural conditions manifest because tiny parasites or intruders invade bivalve mollusks. Then the mollusk attempts to fight it off in self-defense by ejecting nacre many times to cover up the intruder, and this produces a natural pearl. Round pearls are rare, but pearls in other shapes are more plentiful.
Cultured Pearl - The cultured pearl is produced through the intentional, human introduction of foreign objects into the shell of a mollusk.
Baby oysters are usually used for the culturing process. They are placed in plastic cages in protected water for 3 years. A piece of the oyster called .mantle tissue. along with a bead of nacre is pre-formed, and then placed inside the oyster itself. This then produces nacre around the bead. Then they are placed in the sea for 3 more years when the pearls can finally be removed.
There is also a large market for imitation pearls. Some of them are of high quality like the ones from Majorca, and other cheaper ones that made out of either plastic or glass.
References:
Gems: A Lively Guide for the Casual Collector, Daniel J. Dennis Jr., 1999 Harry N. Abrams, Inc., New York, New York
Gemstones: Symbols of Beauty and Power, by Eduard Gubelin and Franz-Xaver Erni, Geoscience Press, Tucson, Arizona
Gems in Myth, Legend and Lore, by Bruce G. Knuth, Jewelers Press, Thornton, Colorado
Gems & Crystals, Anna S. Sofianides and George E. Harlow, Simon and Schuster, New York, New York
WikiPedia Pearl Page
Government Minerals Page
Pearls.com Education Page on Famous Pearls