Pearls are composed of calcium carbonate, which is called nacre, and organic conchiolin. Natural pearl formation is very rare and usually happens when a parasite or other living organism drills its way through the shell of the mollusk. The story that they originate from sand grains is a myth. Almost all pearls currently available on the gemstone market are cultured. Cultured pearls are produced by inserting some tissue or a mother-of-pearl nucleus into the body of a mollusk. Inside this mollusk, in turn, a pearl will slowly develop around the tissue or nucleus. Saltwater pearls come from oysters, while freshwater pearls come from mussels.
The cleaner the surface of a pearl, the more valuable it is. Most pearls never achieve perfection. Some might display abrasions that look like a series of scratches on the surface, or an irregular ridge that looks like a crease or wrinkle. Surface imperfections have less effect on the pearl’s beauty and value if they are few in number, or if they are small enough to be hidden by a drill-hole or mounting.
Nacre, also known as mother-of-pearl, is the substance that creates the iridescent visual effect attributed to pearls. Nacre is an organic material secreted by mollusks over an intruding irritant or implanted nucleus. Nacre thickness is a quality characteristic closely related to luster. If the nucleus is visible under the nacre, or if the pearl has a dull, chalky appearance, one can assume that the nacre is thin. This affects the luster as well as the durability of the pearl. On the other hand good nacre quality often results in attractive luster.
For jewelry with two or more pearls, such as earrings, pearl strands, or other multiple-pearl jewelry, the pearls should match in all the quality factors. If you like more modern styles, look for jewelry designers that deliberately mix colors, shapes, and sizes for unique and fashionable effects.