The Gemmological Institute of America (the GIA) has developed seven pearl value factors and one of those factors deals with shape. There are three main categories of shape – spherical, symmetrical, and asymmetrical. Spherical pearls are round or nearly round.
Symmetrical pearls are those that are made up of exactly similar parts so that if you cut one in half, both halves would be the same. So this category includes button pearls, oval ones, and tear-drop or pear pearls.
Baroque pearls are irregular shaped ones. Sometimes they appear to be shaped as something recognizable, such as an animal or, in the case of the Canning Jewel in the Victoria and Albert Museum in London, as being the torso of a merman (see it at https://collections.vam.ac.uk/item/O33882/the-canning-jewel-jewel-unknown/). Baroque pearls also include mabe pearls and Mississippi pearls.
Pearls also come in many colours but mainly range from gold, cream, pink, to white. Then there are the black pearls, with some lovely variations in colour. The colour can help identify the source of the pearl. Black pearls, for example, generally come from the black lipped pearl oyster, Pinctada margaritifera, which lives in the Indian Ocean and the western and central Pacific Ocean. The colour of the nacre of these oysters is a light grey to black. The pearls from these oysters are known as Tahitian black pearls. They usually display iridescent tinges of purple, green and blue.
Large white, gold and cream pearls come from the Pinctada Maxima oyster which lives in the west Pacific Ocean and the East Indian Ocean. It is used for South Sea pearls. Mid sized white or cream pearls come from the Akoya pearl oyster and are used to produce the Japanese Akoya pearl.
In the early 1990s, the Chinese pearl industry had began to experiment with new techniques to enhance the quality of their freshwater pearls. The pearl mussel, Hyriopsis Cumingii, is used to produce these beautiful freshwater pearls. The pearl colour depends on the colour of the shell and can be white, purple, peach or pink. The pearls can reach sizes bigger than 15 mm in diameter.