fossil fuel vs crude

fossil fuel and crude both are nouns.

Word	Noun	Adjective	Verb	Adverb
fossil fuel	Yes	No	No	No
crude	Yes	Yes	No	No

As nouns, crude is a hyponym of fossil fuel; that is, crude 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
crude: a dark oil consisting mainly of hydrocarbons

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

fossil fuel (noun)	crude (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

fossil fuel (adjective)	crude (adjective)
	not refined or processed
	belonging to an early stage of technical development; characterized by simplicity and (often) crudeness
	conspicuously and tastelessly indecent
	not processed or subjected to analysis
	devoid of any qualifications or disguise or adornment
	not carefully or expertly made