Coal

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 Prov.
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; Prov. 6:28; Isa. 44:19. The words "live coal" in Isa. 6:6
are more correctly "glowing stone." In Lam. 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 Sam.
22:9, 13; Ps. 18:8, 12, 13, etc.) is an expression used
metaphorically for lightnings proceeding from God. A false
tongue is compared to "coals of juniper" (Ps. 120:4; James 3:6).
"Heaping coals of fire on the head" symbolizes overcoming evil
with good. The words of Paul (Rom. 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."