If when you break a chunk apart and it's shiny inside, its coal, not a 'floater'
I'm pretty sure this is spilled coal from ships. Probably Bituminous coal.
Does it burn with a lot of black sooty smoke?? Probably the most important thing is keeping enough air to the coal. If the coal is tossed on top of a wood fire, it will be sitting on top of the wood ash, this will block air to the coal and cause partial burning.
So if your stove has a grate, keep the grate open so air can get to the coal.