Sulphides in coal...

What geologist would be disappointed if Santa Claus left them a stocking full of coal -- if the coal was all full of pretty pyrite?
Yes, Virginia, pyritite is not only caused by hydrothermal processes, but also grows in reducing conditions such as this coal seam. Except as a "pretty rock" I don't think it's ever been mined in its own right for anything of economic use.
This lump of coal is about... 6 cm across. The pyrite is pervasive throughout.