The quality  of a pearl is determined by  various things: the type of pearl, how thick it is, its shine, how clean it is and surface texture, its size, its color, and its shape.

Pearls are one of the very  few organic gems. It is the sole gemstone that has been grown inside an animal. Interestingly, a pearl found in the rough, without ever being touched by man, is already beautiful. It needs no help from man to achieve its own glory.
Now, a pearl isnt perfect, it has certain oddities and abberations, becuz it is organic, and not a cut stone. Simply put, a near perfect string would cost you a fortune.

In recent times, man has become proficient at producing freshwater pearls, and as a consequence they have beomce affordable so that the avergae person now has a chance to possess them.  They are still an organic creation nonetheless and will have all the variance one woudl expect.