black gold

noun4

1. •
   Coal.

   • 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. — 2022
2. •
   (informal) Raw petroleum; crude oil.
3. •
   Black-coloured gold, variously produced by patination, deposition, oxidation, etc. and used in jewellery or in high-technology applications.
4. •
   Black pepper, in the context of being a historically valuable spice.