Cultured pearls quality produced by the mollusc Pinctada margaritifera var. cumingi is determined by a panel of criteria, of which shape is one of the most relevant. Cultured pearls with round and semi-round (RS) shapes are the most valuable. Decreasing the proportion of other pearl shapes (OT: baroque, button, drop, oval), and especially the circled pearls (CL) and rejects (rebuts: RT), is an important challenge for the pearl industry. The present study examined the effect of grafter skill and season of graft on the variation in proportions of CL, RS, OT and RT cultured pearl shape categories in a mono-site culture (Mangareva island lagoon). Six large-scale grafting experiments, carried out in the same way as commercial grafting sessions, were made by seventeen experienced grafters over three seasons. After two years of culture, a total of 42,575 cultured pearls were harvested and sorted into shape categories. This study is the first to apply large-scale traceability to effects of individual grafter skill on cultured pearl shape in P. margaritifera. Grafter effects were clearly demonstrated on the proportions 1) of CL and RS, 2), to a in a lesser extent, of OT and RT, but 3) were low, affecting only a few percent of the overall means. For a high quality cultured pearl production, grafter effect could reduce CL rate by 7% and increase RS rate by 4%. In addition, the results indicate a significantly indirect influence of grafting season on shape class proportions, as these differ in autumn grafts, CL (–12%), RS (+ 13%) and OT (+ 24%), in comparison to winter and spring. Possible implications of "oyster" physiological reproductive status are discussed, as the difference in grafting season corresponds to differences in maturity of the recipient "oysters". Rates of RT were only increased (+ 14%) during the seasons when the lagoon water temperature was the coldest (winter) and the warmest (transition to summer).