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Being a rock-savvy person, are you able to say what it is that's in the middle of the Moeraki Boulders?
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Sorry, I forgot to answer this earlier. The boulders were formed when mineral-rich groundwater saturated a thick layer of mud. I can't recall what the mineral was at this stage - possibly calcium carbonate or something silica-rich. The minerals began coming out of solution (precipitating/crystallising) around shells or other "nuclei", forming concretions. In this case, really big (to 2 metres) boulders, sitting like raisins in a pudding.
Then earth movements fractured the boulders. A new batch of calcium carbonate-rich groundwater infiltrated the mudstone, and crystallised within the fractures. This healed the fractures, so the concretions (boulders) held together.
Then the concretion-laden mudstone was lifted up and eroded. Now and then a boulder emerges from the mudstone.
Some of them break open, revealing the orange calcuim carbonate "seams" (shown above). Others get worn away, and the more resistant calcuim carbonate stands out from the original muddy concretion.
The Maori tale is that they are foodbaskets which washed up on shore and broke open after a canoe of ancients was wrecked.