a precious or semiprecious stone associated with a month or sign of the zodiac and thought to bring luck if worn by a person born in that month or under that sign
birthstone in American English
(ˈbɜrθˌstoʊn)
noun
a precious or semiprecious gem symbolizing the month of a person's birth: the usual list (with modern variants), beginning with that of January, is as follows: garnet, amethyst, bloodstone (aquamarine), diamond, emerald, pearl (alexandrite), ruby, sardonyx (peridot), sapphire, opal (tourmaline), topaz (citrine), and turquoise (zircon)