seacoal

Coal from inside the sea: mineral coal that washes up from the sea onto beaches, from which it can be collected and sold.

(historical, chiefly Southern England) Coal from across the sea: mineral coal, as opposed to charcoal, in a time and place in which the former arrived by ship and the latter arrived overland (such as London in Elizabethan times).

1662, Henry More, An Antidote Against Atheism, Book II, A Collection of Several Philosophical Writings of Dr. Henry More, p. 49:

2020, Ruth Goodman, The Domestic Revolution: How the Introduction of Coal Into Victorian Homes Changed Everything, Liveright, →ISBN, page xvii:

And the change came fast. In 1570 there was still little sign of any major divergence from the traditional use of wood as fuel. Less than forty years later, in 1607, a case brought by the Crown in the Star Chamber stated as fact that ‘sea coal’ — a name for the coal that arrived in London by ship from Newcastle — was ‘the ordinary and usual fuel … almost everywhere in every man's house’. A single generation had made the switch.

(historical, technical, chiefly US) Coal to be used at sea: a certain class of mineral coal, especially suitable for the steam engines of ships at sea and locomotives.

Such coal used in foundry practice, intermixed with foundry sand or applied in a layer on its face, to modify the behavior of the molten metal.