The most common type of coal is anthracite, which has a hardness of 2.2 when compared to the Moh's scale of mineral hardness, according to the Ted Pella website. However, since coal is not a mineral, it is not officially measured in this way.
Generally, the higher the value on the Moh's scale, the harder the material. The Moh's hardness is generally too subjective for anthracite since it does not have a completely uniform internal structure and might be harder or softer in different spots. For example, when a piece of coal fractures, it appears to be softer. The hardest mineral on the scale is the diamond, while the softest is talc.