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In the United States, cobblestones were manufactured large blocks that were impermeable and very durable. Streets were paved with them.

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I think it sounds like a Sett:

https://en.wikipedia.org/wiki/Sett_(paving)

(which is often called a cobblestone)

Edit: Still being laid in Edinburgh...

Here in the Midwest, far away from the granite blocks imported by ships there were companies that made additively "cobblestones", with their names on them, in my walkway there are such stones made in Attica, Indiana. The knowledge of how those were made, is apparently lost. Those things are tough like you wouldn't believe.

Ah, interesting and definitely not a Sett.

I had assumed you meant that the stone had been manufactured into a block, but this is clearly something else.

Some kind of special concrete formulation do you think?

The formula kept as a trade secret, then lost when the company closed or got bought or something I suppose?

Rather unfortunate, alas.

Edited

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