black gold

(crude oil; coal; black pepper): From its black colour and high value.

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black gold (uncountable)

1. (informal, idiomatic) Raw petroleum; crude oil.
   • 1983, James C. H. Shen, “Rejoining the Government”, in Robert Myers, editor, The U.S. & Free China: How the U.S. Sold Out Its Ally‎, Washington, D.C.: Acropolis Books Ltd., →ISBN, →LCCN, →OCLC, page 29:

     With the black gold already gushing from his country's wells, the Iranian monarch did not show much interest in what the Republic of China had to offer in agricultural know-how, commerce, and cultural interflow.

2. Black pepper, in the context of being a historically valuable spice.
3. Black-coloured gold, variously produced by patination, deposition, oxidation, etc. and used in jewellery or in high-technology applications.
4. Coal.
   • 2022 April 20, Christian Wolmar, “Northumberland Line reopening is not the end of the journey”, in RAIL, number 955, page 46:

     They [the railways] are mostly remnants of a long-gone coal industry, laid out largely to bring the black gold from the mines to the waterways.

• See also Thesaurus:petroleum.