Strictly speaking, the gangue is unwanted material which occurs naturally along with or intermingled with the ore being mined and is unavoidably extracted with the Ore. When the two are separated in the first stage of the mineral preparation, the waste material, the tailings, is mainly composed of gangue.
In Coal production the word 'tailings' refers specifically to fine waste suspended in water and the word 'gangue' is never used.
Large-scale operations often leave huge piles of this material on site. Often, as in the case of riverbed metal Extraction, this has buried valuable river-flat soils under the largely worthless and often contaminated tailings. Modern extraction methods have resulted in less damage but Land rehabilitation continues to be a major problem and costly consideration.