The nucleus is also very different. Like the cultured bead, it is almost spherical, but that’s just a fluke. A separate scan of the nucleus itself underscored how different its structure is to the beads of the cultured pearls. Instead of being a solid homogeneous material, it has a tiny hollow center — a cyst formed as a result of damage to the edge of the mantle — surrounded by rays of calcium carbonate terminating in an exterior surface the published paper describes evocatively as “pustolose.” That surface was then wrapped in layer of nacre. That’s how natural pearls form.