Simply put, pearls get their value from their beauty, which stems from their luster, iridescence, shape and clean surface quality. These are called Value Factors, and they are what experts use to evaluate real pearls and determine how much are pearls worth.
Pearls are known for being lustrous gems, and pearl luster — the ability to reflect light and objects like a mirror — is probably the most important value factor of all. Pearls with dull or chalky luster are not worth much, regardless of their size, and pearls with excellent luster are valuable pearls, even if they are on the small side.
Pearls with iridescence — that shimmery rainbow effect visible on the pearl’s surface, also called orient — will be more valuable than pearls without any iridescence at all. Both Hanadama Akoya pearls and South Sea pearls in particular are known for displaying this dazzling quality.
Perfectly round pearls are the most valuable of all pearl shapes, followed closely by smooth, symmetrical tear-drops, near-round button-shapes, then off-rounds, and finally baroque pearl shapes. Round pearls are much more rare, and represent only 5% of any pearl harvest, while baroque pearls can consist of up to 40-60% of a pearl harvest.
Finally, pearls with clean surfaces are higher quality and more valuable than pearls with inclusions like pits, bumps, wrinkles, etc. All pearls will display some kind of surface inclusion, even if it’s so tiny you can only see it under a loupe or a microscope. But all things considered, pearls with cleaner surfaces will cost more.
Other value factors, such as pearl origin (whether natural or cultured), size, and color also play an important part in determining the value of a pearl. However, all things being equal, real pearls with higher luster, more iridescence, less blemishing and desirable shapes will be worth more, even if another pearl is larger in size.