The idea of associating particular gemstones with months of the year goes back centuries, though there has often been some confusion over which gems belong with which month. For example, at various times pearl has been associated with February and November.
From 1912, jewellery makers resolved the differences at a conference and have listed pearl for June ever since. This wasn't a completely arbitrary choice however: the pearl-June link is listed in Hindi texts that pre-date the industry agreement. There is also a traditional poem that reads "Who comes with summer to this earth And owes to June her hour of birth A pearl should wear against her skin // Who's innocence many a heart shall win."
Pearl stands out among the gemstones associated with months for the simple fact that it's not technically a stone. It's the only jewel on the list that results from a living organism.
It's not just June birthdays that are traditionally linked with pearls however. If you take an interest in astrology, pearl is associated with people born under Cancer, which covers June 21 through July 22. If you believe some theories that there should actually be 13 star signs, Cancer would run from 20 July through 10 August. Either way, it's an appropriate match given that the astrological symbol is known as the crab and that Cancer is classed as in the water element category.
There is also a school of thought that links gemstones to the day of the week on which you are born. Under this system, pearl is the choice for Monday babies, which certainly fits in with the old rhyme that suggests such children will grow up ‘full of grace.’ Of course, it doesn't really matter when you are born: the versatility, affordability and elegance of pearls make them a perfect gift for a loved one or for yourself all year round.