How Can I Tell Which Pearls Are Better Than Other Pearls?

A great question.  One that volumes of books have been written to answer.  And even with all those books, it  can still be confusing.  While our answer may be greatly simplified, it will be a good overview.
First, there are really only 3 categories of Pearls:
1.  Shell Pearls.  These are manufactured pearls that use ground shells and other ingredients to create a pearlescent appearance and luster.  They are cast in many shapes: round, near-round, and baroque, knotted and strung, and often embellished with gold clasps.  While they are legally called "Pearls" the industry, generally, does not accept them as such.
2.  Natural Pearls.  Natural Pearls occur in mollusks in both Fresh and Saltwater.  While Fresh Water Natural Pearls of gem quality are somewhat rare, Natural Saltwater Pearls are virtually non-existent.  Many argue that all Natural Saltwater Pearls have long since been harvested.
3.  Cultured Pearls.  These are the mainstay of the Pearl Industry.  Cultured Pearls, like Natural Pearls, are created within the bodies of mollusks.  The mollusk responds to irritants within the shell by coating them with successive layers of Nacre.  In Natural Pearls, the irritant is introduced by nature and may be of any shape.  In Cultured Pearls, the irritant is introduced by man and is round, resulting in most cases in round pearls.    Cultured Pearls may be Fresh or Saltwater.  In both cases the mollusk creates the pearl in the same way.  However, the industry position is that Pearls cultured in Salt Water result in a better quality and more lustrous nacre than the Fresh Water counterpart, although BOTH fresh and salt water have pearls of beautiful quality.  

In the Salt Water Cultured Pearl Category there are essentially 3 types:
a. Akoya Pearls.  So called because MOST of the world's quantity of cultured Salt Water Pears originate in Japan in the Akoya Region.  You may have heard of Mikimoto Pearls - often considered among the best of the best Cultured Pearls.  What isn't routinely known, however, is that Mikimoto Pearls, too, are cultured pearls and are purchased, not produced, by Mikimoto.  What's the difference then?  Well, first, Mikimoto purchases only the best-of-the-best of every harvest of Akoya Pearls.  Literally only the top 5% of Akoya Pearls or better are good enough for Mikimoto.  However, given two strands of top 5% Pearls, what’s the difference between the Mikimoto strand and the other strand?  Believe it or not, it's the Mikimoto Clasp and hallmark.  Take a strand of Mikimoto Pearls and replace the hallmarked clasp with any other, and you have Akoya Pearls.  Without the clasp it’s impossible to distinguish Mikimoto from non-Mikimoto of equally high-quality Akoya Pearls.
b. Tahitian Pearls.  These are the beautiful, large, and expensive multi-colored pearls that are highly coveted by Pearl enthusiasts worldwide.  These Salt Water Cultured Pearls are grown in the area of Tahiti.
c.  South Sea Pearls.  These are the beautiful, large, and expensive white pearls that sell in the range of tens of thousands of dollars - and higher.  From the brilliant whites, to softer creams, to the highly coveted Golden Pearls, there are none better.

    Now, while the above 3-Types of Salt Water Pearls is true, it's important to discuss one additional type of Pearl and that is the "Keshi" Pearl.  The word ”Keshi” is Japanese for ”seed pearl” and indicates a very small irregularly shaped Pearl that was originally produced as a by-product of Japanese Akoya Cultured Pearls.  Today Keshi Pearls can be found in larger sizes, as also the big South Sea and Tahiti Pearl Oysters produce Keshi.
Keshi is virtually all nacre and considering its growth pattern, very close to a Natural Pearl. A Keshi Pearl comes into existence in the Akoya, South Sea, and Tahiti oysters by coincidence when the Pearl Oyster has managed to rid itself of the inserted pearl nucleus, but the process of creating nacre continues. The South Sea and Tahiti oyster will always present either a Pearl or a Keshi. As a difference to this, the Akoya Keshi can be found in the mantle tissue of the oyster and can be additional to the cultured pearl.
Just as Champagne must originate in France to be called Champagne (otherwise it's just Sparkling Wine using the Champagne Method) the name Keshi can only be used for pearls coming from saltwater pearl oysters.  Still, but incorrectly, you will frequently see the term "Freshwater Keshi Pearls."  And don't dismiss this Pearl Type - some incredibly beautiful Keshi Pearl Jewelry is created and is perfectly at home with your other fine jewelry pieces.

Pearl Shape.  Pearl shapes vary from round to near round to baroque.  ALL can be excellent quality pearls, and in most cases shape is a personal preference.  Some prefer completely round as the standard of a well produced pearl, others find the variations in shape attractive and more natural as found in the baroque pearl.
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Pearl Quality.  Now that you know the types, here's a few words on quality.  There are really only 4 discriminants of quality amongst pearls.  Size, luster, nacre, and inclusions.  You may see AAA, AAAA, AAAA, AAAA+ gradings and similar, but these are mostly gradings provided by re-marketers or producers.  Most are surprised to learn that there is NO international standard for pearl grading, and gradings within the industry vary greatly from producer, to wholesaler, to retailer.  

Pearl Color.  So much to be said about color.  white to cream with various overtones such as rose and peach.  The multi-colors of Tahitian Pearls.  The almost legendary Golden South Sea Pearls.  To us, they're all beautiful and we believe color is both subjective and personal choice.

Bottom line.  Fresh and Salt Water Cultured Pearls can both be of excellent quality.  Look for a good size -   6 - 8 millimeters is normal.  Look for a medium to thick nacre - these are less prone to chipping.  Look for a high to very high luster - this is the "Glow" that fine quality pearls possess, and which varies from strand to strand.  Finally, the pearl should be free or mostly free of inclusions - pitting on the surface of the pearl.  Highest quality pearls with have from 0 to 10 percent surface inclusions, depending on pearl type, but less inclusions on pearls, like Diamonds, is always best.