Some etymologies say that the origin of the words "collier" and "colliery" lay in both coal-gathering and charcoal burning, so the prevalence in the Forest of "colliers" and "collieries", rather than mines or pits, may be explained by the considerable amount of charcoal burning that historically went on in an afforested area beneath which lay a considerable amount of coal.

The connotation is blackness, a colley being a black-faced sheep and the commonest collie dogs being substantially black. In some parts of the West Country, blackbirds have been known as "colleys" (which in turn may explain the variant in "The Twelve Days of Christmas" between "colley birds" and "calling birds.")

As Slowhands says, a coal-carrying boat is sometimes called a collier. It is also sometimes called a coaler.

Are colleries and coal mines the same thing? And are colliers and coal miners the same? I grew up in coal mining country and now live in another coal mining area, but have never heard them referred to as colleries. Montana girl

It is I suspect a British idiom - Colliery = Coal Mine & Collier = Coal miner

we also use Collier in a different context to mean a small coal carrying ship !