I tihnk coal should be not so heavy. Better give it the same wight as stone.
1 coal 30 kg
1 stone 7 kg
Coal is actually significantly less dense than most other stones (aprox. half as dense), and realistically should weigh less than stone at the same volume.
Some example densities for comparison:
Coal 1.1 - 1.4
Limestone 2.3 - 2.7
Granite 2.6 - 2.7
Shale 2.4 - 2.8
Hematite (Iron) 5.26
So, realistically, at the same volume of material, you should be able to carry twice as much coal as stone. (and half as much iron as stone, about a quarter as much Copper, and an eighth as much Gold)
This does assume purity of material; Iron, Copper, and Gold ores would likely have a mix of impurities, such as regular stone/minerals in the Ore; but I don't know how much of difference that would make. This seems like a good rough estimate for added realism, without overly complicating things.
At some point, a re-balance of all stone/ore weights to be more in line with realistic densities would be a good idea.