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Geology, Palaeontology & Archaeology / Re: Is This Something To Do With Coal ?
« on: Yesterday at 13:14:07 »Iron work slag is very hard, sort of tungsten carbide hard.
Blast furnace slag has a hardness of 6 to 7 (on the Mohs scale).
https://www.sandblasters.co.uk/iron-silicate-medium-j-blast-blasting-slag-abrasive-media-460-p.asp
Less than hardened steel (7 to , never mind tungsten carbide (9 to 9.5).
This is what PC calls "repetitive antagonism" in his signature.
It is a lot harder than limestone, but ...
Having said that, the knobbly rock in the OP does look like fused ash from a coal fire.