Culm

culm [kulm]
n. inferior anthracite coal; or the waste from anthracite coal mines; slack

[Derives from Middle English colme, meaning coal]

Granted, culm is not an everyday term. If you don't work in a colliery ("a coal mine and all its buildings and equipment"), you don't need to know what it means. Yet coal is a hugely important and impactful fuel for the global economy and environment. Coal is the largest source of fuel for the generation of electricity worldwide, as well as the largest worldwide source of carbon dioxide emissions. Carbon dioxide is a greenhouse gas and the major contributor to an increase in global average temperature and related climate changes.

As a synonym for anthracite, culm makes up one of the three principal categories of coal: the other two being lignite and bituminous.

Anthracite is high-grade hard coal. It is 92-98% carbon. Bituminous is the middle grade of coal. It is softer than anthracite and harder than lignite. Bituminous coal gets its name from an ingredient in it, bitumen ("a mixture of organic liquids that are highly viscous, black, sticky, entirely soluble in carbon disulfide, and composed primarily of highly condensed polycyclic aromatic hydrocarbons"). Lignite is a soft brown fuel, the lowest rank of coal. It is used almost exclusively as a fuel for steam-electric power generation. Lignite's carbon content is approximately 60%.

I came across the word culm in Edmund Morris's biography of Teddy Roosevelt, Theodore Rex:

For eleven weeks, the sheriff of Schuylkill Valley, Pennsylvania, had patrolled the environs of Shenandoah in anticipation of violence. He and his fellow officers sniffed the carbonic gases leaking from untended mines, and avoided the perpetual flames wavering along dark slopes of culm. Valley after anthracite-packed valley seemed to be smoldering with discontent.

The anthracitic word culm has a homonym culm which is completely unrelated. That word -- same pronunciation and spelling -- means "the stem of a grass or similar plant." It came to English from Latin culmus ("stalk, reed"), which originated from the Greek kalamos.