Apart from the 7 rules of judging pear quality, there are also systems of grading that you can use to make your life much easier when you're selecting a pearl at a jeweler. Unfortunately, the pearl industry as a whole has not adopted a universally used standard grading system. Two major grading systems are in fairly widespread use, and used by nearly every pearl retailer: the AAA-A system and the A-D system (Tahitian Pearls system).
These are the most accepted systems, and considered standard by nearly all pearl dealers, both retail and wholesale. But the lack of grading standards has allowed dealers to create their own grading systems that often use superfluous vernacular such as AAA+ and AAAA. The use of "AAAA" or "AAA+" is considered bad taste, or even dishonest, by most reputable pearl dealers. It is seen as a way to simply charge the customer more without providing a superior product.
Note: All Freshwater pearls have a thick nacre, so nacre is not part of the 'AAA-A' grading system.