Exposed to wind, rain and wave action, the spheres erode. The different veins wear at different rates, creating a trellis effect. They have been called turtleback stones. Pip's sandal shows that even after long erosion, they're still a good size.
The Maori tell a story of the wreck of the giant Araiteuru canoe (one of the ancestral waka that brought their ancestors here.) During a greenstone hunting expedition, the canoe foundered, with the loss of everyone on board. The canoe itself formed a reef off shore, and the flax baskets containing food were thrown up on the beach, to become the boulders.