oil. An outsiders' term, not used by those in the business US, 1910.
2
highly potent marijuana. Derived from the previous sense, punning on the richness and the colour of the hashish US, 1946.
3
distilled, concentrated heroin US, 1987
black gold
Crude petroleum oil, so called for its black color and extremely high value. We were barely surviving until Jed discovered black gold beneath our field, and now we're richer than we could ever imagine!