He describes the "thunderstone" as an "agglomeration of brick, soot, unburned coal, and cinder."

The presence of black specks ordinarily indicates unburned coal, and often will be found where the coal contains bone or slate.

As he approached the fire-place a last morsel of unburned coal (or wood) burst into momentary flame, and showed the open doorway.

I reached into the fire pit, pushing my hand deep into the ashes until I found the unburned coals underneath.