When you are going to select a fine pearl, you should consider the factors as below.

Shape: The pearls are in wide variety of shapes. Normally, we can find the following shapes in the market. The more round the shape is and the better the quality is, the more expensive it is.
Round pearls are perfectly spherical. Because of their relative rarity and "classic" nature, they are highly desirable. Round pearls fall into the spherical category.
These pearls are not perfectly round. Instead, they are slightly flattened or elongated, rather than being a perfect sphere. Nonetheless, they are nearly perfect and classified as spherical category.

 
 

These pearls are shaped like an oval - the end is narrower than the center. Ovals are categorized as a symmetrical shape.
 

Button pearls are flattened to some degree, making them resemble a button or perhaps a disk rather than a perfect sphere. These pearls are often used in earrings, where the flattened side can be attached to the setting. Buttons are also categorized as symmetrical.
 

Drop pearls are pear or teardrop-shaped. The drop can either be "long" or "short," depending on its proportions. These pearls make attractive earrings or pendants. This is also a symmetrical shape.
 

These pearls are slightly irregular in their shape. For example, a pearl which might otherwise be considered an oval, button, or drop pearl, but which is not symmetrical in nature, would be considered semi-baroque. Semi-baroque pearls fall into the baroque category of shapes.
 

These pearls are non-symmetrical and irregular in shape. The baroque pearl can be purely abstract in its shape, or it can resemble a cross, stick, or some other shape. Baroque pearls fall into the baroque category.

Size
The size of the pearl greatly depends on the type of pearl. Freshwater pearls range from about 3.0mm to 16mm, Akoya pearls range from about 5mm to 10mm. South Sea pearls and Tahitian pearls can reach size as large as 18mm while its smallest size is about 9mm.
 
 
Color
The general color of a pearl is also called the body color. Typical pearl colors are white, cream, yellow, pink, purple (lavender), silver, or black. A pearl can also have a hint of secondary color, or overtone, which can be seen when a light reflects off the pearl surface. For example, a pearl strand may appear white, but when examined more closely, a pink overtone may become apparent. You could select the color which you like.
 
 
Luster
Pearls produce an intense, deep shine called luster. This effect is created when light reflects off the many layers of tiny calcium carbonate crystals that compose the pearl. This substance is called nacre. When selecting a pearl, consider that the larger the pearl, the more nacre it has, the more luster it exhibits. There is the obvious difference in the amount of nacre when comparing a 5mm Freshwater cultured pearl with a 10mm South Sea cultured pearl.
 
 
Surface
As a mollusk creates a pearl; the layers of nacre do not always adhere smoothly. Sometimes spots and bubbles can appear in the layering process. Pearls with the smoothest surfaces are the highest quality and highly desirable.