Crossword Genius

Mineral forming parts of coral (4)

Ross

I believe the answer is:

coal

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'mineral' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

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(Other definitions for coal that I've seen before include "Three domestic fuels" , "Carbonaceous rock used as fuel" , "Fossil fuel" , "Solid fuel" , "Mined fuel" .)

I've seen this clue in the Evening Standard.
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