Coals - Vine's Expository Dictionary of New Testament Words
Coals
[ 1,,G440, antrax ]a burning coal" (cp. Eng., "anthracite,") is used in the plural in Romans 12:20, metaphorically in a proverbial expression, "thou shalt heap coals of fire on his head" (from Proverbs 25:22), signifying retribution by kindness, i.e., that, by conferring a favor on your enemy, you recall the wrong he has done to you, so that he repents, with pain of heart.
[ 2,,G439, anthrakia ]
akin to No. 1, is "a heap of burning coals, or a charcoal fire," John 18:18; John 21:9.