See Athenaeus 15.57

“Ἀμερίας δὲ γράβιον τὸν φανόν. Σέλευκος δὲ οὕτως ἐξηγεῖται ταύτην τὴν λέξιν: ‘γράβιόν ἐστιν τὸ πρίνινον ἢ δρύινον ξύλον, ὃ περιεθλασμένον καὶ κατεσχισμένον ἐξάπτεσθαι καὶ φαίνειν τοῖς ὁδοιποροῦσιν. Θεοδωρίδας γοῦν ὁ Συρακόσιος ἐν Κενταύροις διθυράμβῳ φησίν: πίσσα δ᾽ ἀπὸ γραβίων ἔσταζεν„

Amerias for torch uses the word γράβιον. And this word is thus explained by Seleucus: γράβιον is a stick of ilex or common oak, which, being pounded and split, is set on fire, and used to give light to travellers. Accordingly Theodoridas of Syracuse, in his Centaurs, which is a dithyrambic poem, says: The pitch dropped down beneath the γράβια, as if from torches.

Usually considered a substratum or balkanic loan word. The word is also found in MGr. γράβος (Epirus), γάβρος (Arcadia). Further compared with Ru. grab 'hornbeam', etc. and OPr. wosi-grabis.