An exceptionally large, lustrous baroque-shaped South Sea cultured pearl from Indonesia was studied by high-resolution X-ray computed microtomography (X-ray μ-CT). The undrilled specimen measured 34.6 mm wide and 40.3 mm long. It was hollow and contained a loose bead. Analysis revealed a large internal cavity filled with liquid, gas, and organic material. The nacre thickness averaged 2.3 mm, and the bead measured 9.1 mm, both well within the typical ranges for South Sea cultured pearls. It is proposed that the cavity was originally occupied by spongy, water-bearing organic material, which inflated the pearl sac and provided a supporting surface for nacre deposition at a distance from the bead. After the nacre coating fully enclosed the organic material, gases were liberated and trapped inside the pearl.