It is by no means certain that the Hebrews were acquainted with mineral coal, although it is found in Syria. Their common fuel was dried dung of animals and wood charcoal. Two different words are found in Hebrew to denote coal, both occurring in Proverbs 26:21, "As coal [Heb. peham; i.e., "black coal"] is to burning coal [Heb. gehalim]." The latter of these words is used in Job 41:21; Proverbs 6:28; Isaiah 44:19. The words "live coal" in Isaiah 6:6 are more correctly "glowing stone." In Lamentations 4:8 the expression "blacker than a coal" is literally rendered in the margin of the Revised Version "darker than blackness." "Coals of fire" (2 Samuel 22:9,13; Psalms 18:8,12,13, etc.) is an expression used metaphorically for lightnings proceeding from God. A false tongue is compared to "coals of juniper" (Psalms 120:4; James 3:6). "Heaping coals of fire on the head" symbolizes overcoming evil with good. The words of Paul (Romans 12:20) are equivalent to saying, "By charity and kindness thou shalt soften down his enmity as surely as heaping coals on the fire fuses the metal in the crucible."
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Coal'". "Easton's Bible Dictionary".