the.com/birthstone

means A gemstone traditionally assigned to the month of a person's birth, worn as a personal talisman or given as a meaningful gift.

from The idea traces back to a much older belief, recorded by the historian Josephus, linking the twelve stones on the biblical high priest's breastplate to the twelve months and zodiac signs. The custom of wearing just 'your' stone instead of all twelve seems to have taken hold in 18th-century Poland or Germany before spreading. The tidy, standardized lists most people know today — one stone per month, no ambiguity — were largely fixed by jewelers' trade associations in the early 20th century, which is a polite way of saying the modern chart was shaped as much by commerce as by tradition.