Not a hot rock
It is very lightweight and has a glassy side and a rougher course side. It rings loud and clear and repeatable on the detector but does ring at all with a pinpointer...found several in a concentrated area of an okd early 1800’s homesite now a corn field
Looks like a cinder to me, partially burnt coal, often called "hot rocks" because they will make a detector sound off. Back in the day folks would spread their cinders from a coal burning stove on walkways when covered in snow or ice. I have hunted places where these things were all over walkways making hunting there quite difficult. The Brits call it "coke", another term for partially burnt coal.