This size of a pearl is measured by determining its diameter in millimeters. Pearls can range in size from 1 millimeter or less to as much as 20 millimeters. The common size for pearls today is 7 to 7.5 millimeters. Akoya pearls that are larger than 8 millimeters are considered rare because the maximum diameter Akoya oysters can produce is 10 millimeters. Pearls that are produced to have a diameter greater than 10 millimeters are usually Tahitans, South Sea, or freshwater pearls. The larger the pearl is, the more value it has.