An average pearl will take two to three years to develop. The larger pearls can take up to 5 years to completion. With time comes difficulty with the pearl developing into a symmetrical gem. As a result, very large, round or near round pearls are extremely rare and can command a high price. High quality pearl necklaces composed of perfectly round pearls are therefore very valuable.
International standards for sizing strands of pearls provide for a 0.5mm tolerance between Akoya pearls. In other words, where a strand has been advertised as 6.5mm, the actual finished size will be between 6.0mm and 6.5mm. Freshwater pearl standards provide for a 1.0mm tolerance so an 8.0mm necklace would range in size from 7.0mm to 8.0mm. South Sea Pearls and Black Tahitian Pearls are a little more exacting in that each pearl should be measure to the 0.1mm. For example a Tahitian pearl strand could be sized from 8.3mm to 10.8mm.
If strung correctly, the larger pearls, say 6.5mm, will be strung in the center of the strand while the slightly smaller ones, 6.0mm, will be strung closest to the ends or clasp. Even though the size is slightly graduated from the middle to the ends, it is not considered a graduated necklace since it is within the tolerances of a uniform strand. This slight variation in pearl size is not normally noticeable to the untrained eye.
Large pearls can make outstanding pieces of unique pearl jewelry. See Pearl Sizes Chart for Size Comparison