There is no such thing as green coal.
coal
he turned the coal white
Coal is formed from plants, which turned into peat, then lignite, then coal.
it already is coal is fossilised wood
By burning them
by heating the absence of air
By burning the coal. No, this is not necessarily the only way. Other ways are very important.
Millions of years of pressure and heat turned decaying animals and plants into coal.
when coal burns the energy change is chemical -------------> electrical
If your stone turned green it probably has moss growing on it.
From prehistoric plants that died and turned to coal. Sometimes in underground coal mines we find a tree stump that is coal, but can still be recognized as a tree stump.
Coal can't be turned back into coal ore once it is mined. Since there is no enchanting in Minecraft PE, there is no way to obtain coal blocks in survival.