The size of a pearl is one of the GIA’s 7 factors of pearl quality. So how is the size of a pearl measured? With round and near-round pearls (spherical pearls), it is quite easy. You need to measure the diameter of the pearl, usually to the nearest 0.5th millimetre. So a pearl might be 5mm or 10mm in diameter. The seed pearls in the necklace on the left in the photo at the top of the post are tiny and measure 1.6mm. The South Sea pearls in the graduated necklace below range from 12mm to 15mm in size. This is a normal size range for this type of saltwater pearl.
Akoya pearls, Japanese pearls which resulted from techniques for culturing pearls developed by Kokichi Mikimoto, are a much smaller pearl, as the oyster used is a much smaller oyster than that used for South Sea pearls. The main pearl used in cultured strands of pearls since the 1920s, Akoya are rarely bigger than 10mm and normally in the 4mm to 6mm range. The opera length strand of Akoya pearls below are 7mm.
Pearls that are symmetrical (pear-shaped, for instance) and asymmetrical (irregular-shaped like baroque pearls) need to have their diameter and their length measured to determine their size. The South Sea pearls below are pear shaped and need to be measured this way. They vary a bit in size, with the large ones being about15mm in length to 13mm in diameter.