Some strange rocks

[ugotit22]

Hey guys
Need some help from the experts. Was out metal detecting yesterday and old construction site. Found a bunch of these rocks. My first thought was they were large pieces of colonial glass. But after cleaning up a bit idk.
Are these rocks, something man made?
Found near Amenia NY.

Thanks in advance.
Keith

 

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[ugotit22]

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[Indian Steve]

I believe that they are slag. Are there any old iron furnaces nearby?