Pearls are measured in diameter in units of millimeters. Freshwater pearls usually measure between 4.0 mm - 10.0 mm, as seen on the left. All else equal (luster, surface quality, shape, etc.), large pearls do command a much higher value than small pearls. The large pearl as seen on the bottom took 6 years to create. Good roundness is contingent on the mollusk itself repeatedly making consistent and uniform coatings over the same pearl for however long the farmer nucleates. It's a great risk to let the nucleation continue for so many years, as one bad storm could destroy an entire crop in a matter of hours. For the pearl farmer, there is also considerable financial investment involved in waiting longer to harvest the pearls and take them to market.