crushed rock vs ballast

crushed rock and ballast both are nouns.

Word	Noun	Adjective	Verb	Adverb
crushed rock	Yes	No	No	No
ballast	Yes	No	Yes	No

As nouns, ballast is a hyponym of crushed rock; that is, ballast is a word with a more specific, narrower meaning than crushed rock:

Other hyponyms of crushed rock include bank gravel, pit-run gravel, pit run, shingle.

crushed rock (noun)	ballast (noun)
rock fragments and pebbles	any heavy material used to stabilize a ship or airship
	an electrical device for starting and regulating fluorescent and discharge lamps
	a resistor inserted into a circuit to compensate for changes (as those arising from temperature fluctuations)
	an attribute that tends to give stability in character and morals; something that steadies the mind or feelings
	coarse gravel laid to form a bed for streets and railroads

crushed rock (verb)	ballast (verb)
	make steady with a ballast