The answer is we do not know for certain. But we do know that they did not arrive as ballast in ships, as is often said. The whaling ships and small trading vessels that arrived in Nantucket in the nineteenth century were laden with whale oil and all manner of manufactured goods and raw materials that the islanders needed to live here. Only rarely did a vessel arrive “in ballast” with no cargo aboard. As well, the random rocks that might be used for ballast would not make good paving.
Main Street was paved with cobblestones beginning in 1836 or 1837, although there is evidence that some paving might have started in 1834. Oblong rocks with generally flat tops were carefully selected to create a durable paved surface. The rocks were laid upright, with their narrower ends driven well into a layer of sand bedding. There is no source on Nantucket for large numbers of such rocks, so it is likely the bulk of the island’s paving cobbles were imported from the mainland when needed.
In 1954, R. Newton Mayall created a map for the Nantucket Information Bureau that claimed that Main Street was “paved with cobblestones in 1837 brought from Gloucester.” Nantucket historian Will Gardner contacted a local historian in Gloucester in 1961 who confirmed that there was a huge cache of cobblestones located on the Gloucester waterfront at that time and that cobbles from there had been shipped to Boston in times past, but he could not say whether cobbles had also been shipped to Nantucket from that port. As Nantucket in the 1830s was well connected by coastal trading vessels to ports up and down the east coast, the Main Street cobbles could have come from almost anywhere. In the absence of further evidence, we can only speculate.