The iridescence-lacking pearl is regarded as a low-quality product because it shows no iridescent color which is generated by the interference of the light reflected at the organic-inorganic boundaries in the regulated interstratification of organic sheets and thin aragonite tablets. In this study, we investigated the microstructural difference between normal and iridescence-lacking pearls, as well as original nacreous layers in the shell of the pearl oyster, Pinctada fucata. Cross-sectional observation by scanning electron microscopy revealed abundant organic spherules of a few hundred nanometers in diameter attached to the inter-crystalline organic sheets in the iridescence-lacking pearl. The incoherent light scattered by the spherules inhibit the formation or emission of the iridescent color. The same spherules were also observed in hazy nacreous layers of the shell. The organic spherules often connected to the gap of inter-crystalline organic sheets implying that the spherules consist of same components of the organic sheets. Their abundance varies along the thickness of nacre, suggesting that their formation is determined by environmental factors, as well as genetic ones. (C) 2015 Elsevier B.V. All rights reserved.