Talc.
it is 6 1/2 in the mohs hardness scale
Talc (1 on the Mohs Hardness Scale) can be scratched by a fingernail.
Galena is listed at 2.5 on the Mohs scale of mineral hardness.
Coal, as an organic sedimentary rock, is not determined to have a hardness on the Mohs scale of mineral hardness.
Diatomite has a hardness of around 5.5 on the Mohs scale of hardness.
It's called the Ore
The hardness of Limonite is 4 t0 5.5 on the mohs hardness scale.
Azurite has a hardness of 3.5 to 4 on the Mohs hardness scale.
flint is a type of quartz, so it has a hardness of about 7 on the mohs scale of hardness
Chalcopyrite has a hardness of 3.5 to 4 on the Mohs scale.
The Mohs standard hardness scale that was developed by Friedrich Mohs (1773 - 1839).
Nickel has a hardness of 4 on the Mohs scale.