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

shape
The pearls are a variety of shapes. Usually, we see the shapes round, drop,button and abnormity in the market. The rounder of the shape , the better for the quality, and the price is more expensive.
At Popearl,We offer the highest quality, rarest pearl shape round. Shapes that are not spherical or even symmetrical are considered lower quality. Akoya, Tahitian, and South Sea pearls found in jewelry have a tendency to be the roundest, while Freshwater pearls are drop ,button and slightly off-round. Size
The size of the pearl greatly depends on the type of pearl. Freshwater pearls range in size from about 3.0mm to 16mm, Akoya pearls range from about 5mm to 10mm, and South Sea pearls and Tahitian pearls can reach sizes as large as 18mm whitle 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,silver, or black. A pearl can also have a hint of secondary color, or overtone, which is seen when 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, so it will also exhibit even more luster. Compare a 5mm Freshwater cultured pearl with a 10mm South Sea cultured pearl and the difference in the amount of nacre is obvious. 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, most popular pearls.