1) Coal is a sedimentary rock.
2) Coal ASH has had all the carbon burned out of it, and has been melted and cooled. It would constitute a sort of synthetic metamorphic rock
I’m not clear on where you studied geology, but it seems to have been deficient.
Coal is a sedimentary rock.
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By its nature coal is a banded material which makes it weak by comparison to most other rocks. Intact rock strength is commonly defined as the strength of the rock material that occurs between discontinuities.
In this case sedimentary material = compressed vegetation to a hardness that varies between the friable lignite and sub-bituminous, bituminous and anthracite, a 2.75–3 on Mohs, comparable to Talc, Gypsum & Calcite.
You can crumble low grade coal in your hands; hard coal shatters when hit with a ordinary hammer. Granite when hit with a hammer makes a very small scrape, if that.
Granite a common volcanic ROCK is a 6 on the same scale.
Apatite, Phosphorite and Phosphate ROCK is a 5 on the same scale.
Even though its nominally labeled a rock, coal and volcanic rock are not the same thing. Each has different properties.