According to a simple principle, a stone is assigned to each month of the year. Over time, some months have been assigned several stones.
In the beginning... was Aaron
Since ancient times, stones have been allocated according to the breastplate of Aaron - brother of Moses - described in the Bible. Each stone in this breastplate is attributed to one of the twelve tribes of Israel.
Here's where the first list of stones came from, which later served as the basis for the birthstones we now know: sardony, topaz, emerald, carbuncle (the name given to garnet), sapphire, diamond, opal, agate, amethyst, chrysolite, onyx and jasper.
From zodiac to calendar
Over the following centuries, a link between the twelve stones of this pectoral and the twelve signs of the zodiac was mentioned by several cultures and civilizations. It is in these writings that the potential powers of the various stones are evoked, powers linked to the astrological sign to which the said stone was attached.
With the creation of the Gregorian calendar in 1752, stones were assigned to months rather than zodiac signs. An official list was established, although there are often variations from one place to another. The idea of wearing jewelry with a birthstone dates back to the 18th century, with the arrival of numerous gem traders, notably in Poland. The principle of birthstones greatly benefited the trade of the time, with jewelers not hesitating to seize on the idea.