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.
< sigh >
Tell me something I don’t know.
The discussion centers on coal ASH, not on coal.
COAL ASH is a mixture of silicates and metal oxides. The carbon has all been burned out. Tell me, without looking it up: what is the chemical composition of granite? Do you have the slightest idea?