Looks like a cinder, which is a piece of burned coal, or partially burned coal. These are common in some areas and often called "hot rocks" because they set off a metal detector. There are other "hot rocks" which can be ores such as iron which also will set off a detector. In the old days some folks would throw the cinders from their coal burning stoves on icy walkways to provide traction. I have hunted old home sites where it was impossible to hunt along the walkway because there were so many cinders setting off the machine. Some folks still use coal for heat, and I'm sure they still throw the cinders on icy walkways, I did in the early 1970s. It is the high carbon content of cinders, and also the old dry cell battery posts which cause the detectors to think there is metal there.
