1) Originally used for charcoal as well as for the 'stone' extracted from underground and used as fuel.
The word is of Old English origin and it occurs in documents from the early ninth century. It has diverse but related meanings and from c.1200 is on record as the name for the fuel that is produced when wood has been burnt, that is charcoal: from roughly the same period it was the word also for the ‘stone’ that miners extracted from the ground, commonly used as domestic fuel and by smiths and lime-burners. In many accounts the plural ‘coals’ was used in both cases. In that early, overlapping period the latter was commonly called sea-coal, albeit often in Latin: 1292 In carbonibus marinis emptis xls iiijd, Bolton Priory
1304 minere carbonum marinarum, Masham
1371 ij schaldres de secole, York. Later, where ambiguity might arise, the context often holds the key to our understanding: 1457 it shall be lefull … to gete and have fuell … for the fyre as of stone or cole, West Bretton
1638 pay the several wages to them due for … leadeing of charcoale … and restore to the poore menn their Tallyes of the coales ledd, West Riding. The verb ‘to coal’ usually meant ‘convert into charcoal’: 1457 the woddes of the seid mese shall be coled, West Bretton
1466 the said woods to cut downe, coale and springe, Denby Dale. Sometimes, though, it meant to extract coal from the ground, perhaps illegally: 1709 believing him to have a hand in their coaling my ground, Whitley.