- 1). Measure the diameter of round pearls. Use the measuring tape to measure across one face of the pearl in millimeters.
- 2). Measure fancy pearls, or pearls that are other than round-shaped, by measuring both the diameter and length of the pearls. Measure the diameter at the widest part of the pearl and the length from the top to the bottom.
- 3). Express measurements in millimeters. For pearls that measure fewer than 10mm, round to the nearest 0.25mm. For pearls that measure larger than 10mm, express the size in the nearest 0.1mm.
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