Coal Question

Spring has arrived in the Deep South.
To mark the occasion, I set out this afternoon to crush coal needed to feed the On3 coaling tower I built last year and for six drop bottom gons which will service it.
That's a lot of coal in any scale.
Anyway, I have accumulated lumps of coal in my travels over the years for just this purpose. Some of it came from Chama, some from Durango, some from odd places along the line here and there, but always on either the D&S or the C&TS.
I have read here that there are differing grades of coal, how it burns, etc.
Among my samples, there were some lumps that were a deeper, rich black color than others, and these seem to be much softer than others. Very easy for me to work with in comparison.
At the other extreme, there were some that were much more dense, much harder, and somewhat lighter in color.
Which of these two types is preferred? I would guess that the blacker, softer coal burns better, and that the other stuff is the kind that tends to burn with more difficulty and form clinkers unless you really watch it. Am I right?
Our previous discussions on coal have been informative, but I'll bet anyone who has fired this stuff knows by looking at it what sort of day they will have.
Thanks,
Mike