This is the 17687th most frequent Russian word.
угольной
"Угольной" means "coal" or "carbon-related" (adjectival form).
The usage of "угольной" here refers to coal-derived, in describing the pencil material.
Он посмотрел на угольной графит карандаш.
He looked at the coal graphite pencil.
The term "угольной" is an adjective related to the coal industry.
Фабрика производит материалы угольной промышленности.
The factory produces materials for the coal industry.
The adjective "угольной" modifies "шахте" to indicate it is a coal mine.