precious stone vs pearl

precious stone and pearl both are nouns.

Word	Noun	Adjective	Verb	Adverb
precious stone	Yes	No	No	No
pearl	Yes	No	Yes	No

As nouns, pearl is a hyponym of precious stone; that is, pearl is a word with a more specific, narrower meaning than precious stone:

precious stone: a precious or semiprecious stone incorporated into a piece of jewelry
pearl: a smooth lustrous round structure inside the shell of a clam or oyster; much valued as a jewel

Other hyponyms of precious stone include crown jewel, solitaire, diamond, ruby, emerald, sapphire.

precious stone (noun)	pearl (noun)
a precious or semiprecious stone incorporated into a piece of jewelry	a shade of white the color of bleached bones
	a smooth lustrous round structure inside the shell of a clam or oyster; much valued as a jewel
	a shape that is spherical and small

precious stone (verb)	pearl (verb)
	gather pearls, from oysters in the ocean