We know that the larger the size, the higher the price. But price jumps between pearl sizes are often uneven. As the size reach the 8 - 9mm or above, pearl prices tend to increase much higher.
According to CIBJO (The world Jewelry Confederation) The Pearl Book, different pearls are measured by different methods with different minimum units' expression.
Pearl size measuring methods:
Pearl Size Description
Round shape: average diameter
Fancy shape: maximum diameter, maximum length
Pearl size: < 10mm, expressed to the nearest 0.25mm
>,=10mm, expressed to the nearest 0.1mm
Maximum diameter of the largest pearl and the smallest pearl on the strand
Pearl size:expressed to the nearest 0.1mm
Stated as the minimum size
Pearl size:expressed to one half(0.5)mm variations