Easton's Bible Dictionary (1897)
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. ” The latter of these words is used in Job 41:21; Prov. 6:28; Isa. 44:19. ” “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.
Smith's Bible Dictionary (1863)
The first and most frequent use of the word rendered coal is a live ember, burning fuel. (Proverbs 26:21) In (2 Samuel 22:9,13) “coals of fire” are put metaphorically for the lightnings proceeding from God. (Psalms 18:8,12,13; 140:10) In (Proverbs 26:21) fuel not yet lighted is clearly signified. The fuel meant in the above passage is probably charcoal, and not coal in our sense of the word.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
COAL . There is no evidence that the Hebrews were acquainted with coal. They used charcoal for their fires. The Hebrew words which are translated "coal" etymologically refer to heat in general, usually to fuel of wood, and in 1 Kgs 19:6 and Isa 6:6 to hot stones. T. the Greek words, Rom 12:20 and John 18:18; John 21:9, refer likewise to charcoal.