Here some of images of different sizes of pearls.
The size of round saltwater cultured pearls is expressed in terms of their diameter measured in millimeters. One millimeter is about 1/25 of an inch.
The size of non round pearls can be expressed in terms of their greatest width and length and in some cases depth. The measurements are generally rounded to the nearest half or whole millimeter.
* Images are found on google & Pearl Buying Guide
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