The pearl is the only gemstone which is 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, entering the soft mantle tissue, and picking up epithelial cells. In response to the irritation, the epithelial cells form into a sac (known as a pearl sac) which secretes a crystalline substance called nacre (pronounced nay-ker). Nacre is the same substance that makes up the interior of the oyster's shell, and builds up in layers around the irritant, forming the pearl.
There are approximately 8,000 different species of two-shelled (bi-valve) mollusks, of which only about 20 are capable of consistently producing pearls. Natural pearls have always been extremely rare and valuable. 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 rarer still, and are highly prized. Most natural pearls are irregularly shaped.
In a completely natural state, only a very small percentage of oysters will ever produce a pearl at all. Of the pearls which are produced, only a handful will develop into a desirable size, shape, and color, and only a small fraction of those will be harvested by humans.
It is commonly assumed that only one in ten thousand oysters will naturally produce a gem quality pearl. Obviously, if we relied only on nature, ownership of pearls would still be relegated to only the wealthiest people in the world, and pearl producing oysters would be on the brink of extinction due to over-harvest. As pearls have been a prized gem by much of the world's population for thousands of years, this need has led to the development of cultured pearls.
In the late 19th and early 20th centuries, several Japanese researchers discovered a method of dramatically increasing the number of oysters that produce pearls. Essentially, the technique involves inserting a foreign substance, or nucleus into the tissue of the oyster or mollusk, then returning the creature to the sea and allowing the resulting cultured pearl to develop naturally. A technique for artificially stimulating the development of round pearls in Akoya oysters was perfected and patented in 1916. Since then, this technique has been improved upon and used extensively throughout the pearl industry – no longer simply used to culture Akoya pearls, but freshwater, South Sea and Tahitian pearls as well.
This development greatly expanded the pearl industry, in which pearls could be farmed like an agricultural crop, rather than simply sought hit-and-miss. These cultured pearls can now be produced in sufficient quantities to make them available to virtually everyone.
The cultured pearl industry has now far surpassed that of the natural pearl industry. Although a market still persists 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 a pearl necklace from nearly any store in the world means purchasing a strand of cultured pearls.