The Laboratoire Français de Gemmologie (LFG) once examined a parcel of nearly 100 pearls submitted as natural pearls from Pinctada maculata. The owner, Bruno Arrighi of Croissy Pacific, had collected them himself in French Polynesia over the prior year (figure 1).
Pinctada maculata is a small bivalve mollusk (figure 2) found across the Pacific, especially around French Polynesia and the Cook Islands. It produces the rare "poe pipi" — better known simply as pipi. Attempts to culture pipi failed in the 1950s, and none since have stuck, so every pipi on the market today is treated as a natural pearl. That alone sets it apart from the Tahitian pearls most people associate with French Polynesia, which come cultured from the much larger black-lipped Pinctada margaritifera.

Finding pipi is slow, low-tech work. In one method, divers gather the largest Pinctada maculata shells and leave them in buckets of seawater on the beach to putrefy. After about three days the valves are sorted, and only those with blisters are kept; settled at the bottom of the bucket, a diver may find a few loose natural pearls. The older technique, kakaro in the Paumotuan language, skips the wait entirely — shells are opened underwater during the dive to look for pearls on the spot.
Most pipi run from orange to cream, gray and white, but the prized color is a deep golden. Sizes are tiny, generally 1 to 4 mm, which is part of why a fine matched line is so hard to assemble. The standout in figure 1 carries one of the best golden colors possible and measures 9.6 mm across — which the contributors believe is the largest pipi ever documented.
Microradiography of these pearls showed the internal structure typical of natural pearls: onion-like stacking of aragonite layers, sometimes around a calcitic core. Raman scattering, UV-visible reflectance and UV luminescence have helped pin down the mollusk species in other cases, but for Pinctada maculata the results were not conclusive — so the exact source mollusk could not be confirmed with certainty.

Source: GIA.edu