Combustible rock.

1400 years ago Arabs burned wood for fire however they didn't know about stones that burn.

What is Coal?
Coal is an organic sedimentary rock that forms from the accumulation and preservation of plant materials, usually in a swamp environment. Coal is a combustible rock and, along with oil and natural gas, it is one of the three most important fossil fuels. Coal has a wide range of uses; the most important use is for the generation of electricity.
Geology, Coal, 2020

"Coal is a combustible rock" however it was not known to Arabs 1400 years ago.

Saudi Arabia produces virtually no coal for its own energy use. It chooses instead to import 123.4 thousand short tons per year, with that value increasing almost every year throughout the 2010 decade. Saudi Arabia consumes only a small amount of coal for its energy use per year as of 2016 (123 thousand short tons per year), which fulfills less than 0.1% of its energy needs.
Climate Scorecard, Saudi Arabia Increases Coal Production, 2019

Combustible rock was not known in Arabia 1400 years ago, however it was portrayed in the Quran.

[Quran 2:24] But if you do not-and you will not-then beware the Fire whose fuel is people and stones, prepared for the disbelievers.

Stones that are fuel for fire, which means they are combustible stone. Today we know that coal is combustible stone.

How could an illiterate man who lived 1400 years ago have known about stones that burn?

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