Contractors destroy old cobblestones

Daphnée Hacker-B. says in the Journal that road contractors are destroying old cobblestones that they find while redoing roads. But she says here that those old blue-gray cobbles are made of granite, which is not so. I went on an archaeology tour a few summers back where they told us the blue “stones” are made of slag from iron foundries. Here’s an example from another place showing blue iron slag cobbles just like ours. So the “stones” aren’t really stone, although whether they’re worth preserving is another matter.