First of all, a photo of my "stash"

Next, take a small piece with a lot of shiny surfaces (this is from the coal maceral vitrinite, based on the Stopes-Heerlen coal classification):

Place the piece in a plastic bag and take out a small hammer:

Gently break up the lump into smaller pieces, aiming to have the largest piece no more than 1 scale foot across (7mm in 0 gauge, 4mm in 00 gauge). A foot wide lump of coal is not unusual though the fireman may want to break it up into smaller pieces.

Tip out the broken coal and examine. Take any large pieces out and place back in the plastic bag for a second go round. Small pieces can be sieved out or used as a bed for the larger pieces to glue to in the tender, etc.

And there is a nice pile of coal, ready to place in the tender, bunker or coal wagon. Glue down with the usual watered down white glue.

Note: not all coal is shiny but this does look better on a model. Coal classification is based on the content of the coal (i.e. the basic ingredients that went into making the coal) and the rank of the coal. Low rank coals will be dull (lignite so dull and friable it was rarely used to fire locomotives) up to anthracite which is very low in water content and high in carbon. Welsh steam coals, famous for fueling Castles and Kings, came from the Rhonda Valley. Coal rank increased from east to west across South Wales from Bituminous to Anthracite, with the Rhonda Valley located roughly midway between the extremes. Ash content was a major in determining the calorific value of a coal - low grade coals were often high in ash content leading to poor steaming. Inferior coal was often a mixture of good coal and carbonaceous shale - I can remember my parents complaining about too much "slack" n a coal delivery.