seacoal (n.)

also sea-coal, old name for "mineral coal" (as opposed to charcoal), mid-13c.; earlier, in Old English, it meant "jet," which chiefly was found washed ashore by the sea. The coal perhaps so called from resemblance to jet, or because it was first dug from beds exposed by wave erosion. From sea + coal. As it became the predominant type used, the prefix was dropped.