The following information has been obtained from Pearl-Guide.com, which takes you deeply into the world of pearls. Established in 2003, Pearl-Guide.com has quickly grown into the world's largest pearl information source with more than 30,000 pages of pearl-related content.
Pearls have been prized for their beauty and rarity for more than four thousand years. From ancient China, India, and Egypt, to Imperial Rome, to the Arab world, to Native American tribes, cultures from around the world and throughout recorded history have valued pearls longer than any other gem.
Pearls are the only gemstones grown inside of a living organism. Pearls are formed within oysters or mollusks when a foreign substance (most often a parasite - not a grain of sand) invades the shell of the mollusk and enters the soft mantle tissue. In response to the irritation, the mantle's epithelial cells form a sac (known as a pearl sac) which secretes a crystalline substance called nacre, the same substance which makes up the interior of a mollusk's shell, which builds up in layers around the irritant, forming a pearl.
There are approximately 8,000 different species of bi-valve mollusks, of which only about 20 are capable of consistently producing pearls. Natural pearls are extremely rare. Because the layers of nacre tend to maintain the irregular shape of the original irritant, natural pearls which are round or spherical in shape are even more rare. Most natural pearls are irregularly shaped.
In a completely natural state, only a very small percentage of mollusks will ever produce a pearl and only a few of them will develop a desirable size, shape, and color; only a small fraction of those will be harvested by humans. It is commonly assumed that one in ten thousand mollusks naturally produce gem quality pearls. Obviously, if we relied only on nature, ownership of pearls would still be relegated to the wealthiest and pearl producing mollusks would be on the brink of extinction due to over-harvest. As pearls have been prized for thousands of years, this need has led to the development of cultured pearls.
In the early part of the 20th century, Japanese researchers discovered a method of producing pearls artificially. Essentially, the method involves inserting a foreign substance, or nucleus, into the tissue of the oyster or mollusk, then returning it to the sea, allowing a cultured pearl to develop naturally. This practice was already quite widespread culturing hemispherical pearls known as mabe pearls. Kokichi Mikimoto is credited with perfecting the technique for artificially stimulating the development of round pearls in akoya mollusks, receiving a patent for this technique in 1916. Although patented in 1916 this technique has since been improved upon and used extensively throughout the pearling world - no longer simply used to cultured akoya pearls, but freshwater, South Sea and Tahitian pearls as well.
The cultured pearl industry has now far surpassed that of the natural pearl industry. Although a market still exists for pearls gifted to us by nature, these pearls are becoming more and more difficult to find, with rare full strands being auctioned for hundreds of thousands of dollars. Today, purchasing pearls from nearly any store in the world means purchasing a strand of cultured pearls.
Freshwater pearls cover a wider range of color and shape than any other pearl type. Originally these pearls were marketed as a lower quality, inexpensive alternative to Akoya pearls. For this reason, jewelry designers traditionally used freshwater pearls for pieces that favored design over the gems themselves. However, since the 1990s, a class of freshwater pearls that are round and lustrous have emerged which rival Akoya pearls in both quality and value. Freshwater pearls cover the widest range of options for pearl buyers in size, shape and color.
Unlike cultured saltwater pearls, freshwater pearls are not bead-nucleated and are therefore typically less round. Instead of using a bead, a small square of mantle tissue is cut from a donor mussel's inner lining and inserted into the lining of the harvesting mussel to instigate pearl formation.
The unique harvesting technique makes freshwater pearls purely composed of nacre. Nacre is the biological term used to describe the mother of pearl substance that gives pearls their distinguishing glow. It also is one of the main value factors for grading all pearl types. Because cultured saltwater pearls are bead nucleated, their nacre thickness ranges between 0.2 mm and 4.0mm.
Only two percent of all freshwater pearls are round or near round. Though this number seems quite small, the abundance of freshwater pearls is much greater than other pearl types. Each freshwater mussel will produce up to 40 pearls, whereas one saltwater mollusk usually only produces one or two pearls.
The most common shape in freshwater pearls is oval or button shaped (sixty percent). Thirty-eight percent are Baroque and Semi-baroque. Almost all Freshwater pearls come from China. Since the 1990s, Freshwater pearl quality has steadily increased. Though you never will have a perfectly round Freshwater pearl, the likelihood of finding a quality near round pearl is much greater today.
Potato Shaped Pearls are also referred to as semi-round or off round pearls, which are side drilled without any hard edge or flat side.
Stick or Biwa Pearls
Biwa or stick pearls look like sticks, they are natural free shaped pearls with rugged surface. They can be drilled in both directions - end to end, or side to side.
Baroque pearls are simply pearls that have an irregular shape. Cultured freshwater pearls are most commonly baroque.
A freshwater cultured pearl with a crinkled surface and elongated shape, such that it resembles a grain of rice.
These are freshwater pearls that are named for their peanut-like shape.
Button pearls are pearls in button shapes. They can be either bottom half-drilled as individual pearls, as the first picture shows, to be used for pendants, earrings or rings or they can be side drilled.
Pearl shape where one side of the pearl is flattened. Also referred to as mabe pearl, which is a hemispherical shaped pearl that's grown against the inside of the oyster's shell, rather than within its tissue.
Keshi pearls are small non-nucleated pearls typically formed as by-products of pearl cultivation. A Japanese word also meaning "poppy", it is used in Japanese for all pearls that grew without a nucleus.