In the day of the rule of Natural Pearls, truly 100% round shaped pearls were once very difficult to obtain, thus they commanded a much higher prize. Because of this scarcity of round pearls, the most valuable shape was that of a tear-drop (the name "pearl" is thought to originate from the Spanish-Italian word "pera": pear), but button-shaped pearls were used to provide the illusion of a round pearl (when properly set these pearls look round).
Later on, with the appearance of the cultured pearl, the perfectly spherical shaped pearl became much more common. It was said that only from 5 to 25% of a pearl harvest came out round to semi-round in shape, but nowadays that number has increased to over 90% (due to a decreasing of the nacre thickness). Due to the ancient shape and value considerations (originated with natural pearls), a round shape takes first place in value, followed by "off-round" (about 2-5% of the pearl's shape is not-round), semi-baroque (symmetrical shapes: drops, buttons, ovals) and finally baroque shapes (asymmetrical pearls). Depending on the source, circled pearls may be considered in their own group (circlé) or as part of the semi-baroque group. Circled pearls may have one or more "belts" or constrictions that will give the pearl a very unique appearance.
Semi baroque pearls are highly coveted in some countries, especially in the drop shapes, where they are known as "cabochons", "tears" or "pears". They are very popular in drop-style jewelry and used mainly in pendants and earrings. Baroque pearls do not have a definitive shape, being always irregular in their skin/surface, coloring and appearance. Sometimes, keshi pearls are easily mistaken with baroque pearls, the main difference being the presence of a nucleus in the latter. Baroque pearls are very unique, so they can give a more natural look to jewelry.
So, as you can see, many pearl shapes exist, but today's peoples tend to identify a pearl as being "round". This is due to the fact that the Japanese were the World's leading suppliers of cultured pearls (today, the main producer of pearls is China), and they are able to produce many round pearls thanks to: