fossil fuel vs rock oil

fossil fuel and rock oil both are nouns.

Word	Noun	Adjective	Verb	Adverb
fossil fuel	Yes	No	No	No
rock oil	Yes	No	No	No

As nouns, rock oil is a hyponym of fossil fuel; that is, rock oil is a word with a more specific, narrower meaning than fossil fuel:

fossil fuel: fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content
rock oil: a dark oil consisting mainly of hydrocarbons

Other hyponyms of fossil fuel include coal, gas, natural gas, crude, crude oil, fossil oil, oil, petroleum.

fossil fuel (noun)	rock oil (noun)
fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content	a dark oil consisting mainly of hydrocarbons