German jewelers or Polish gem traders may be responsible for creating the original list of birthstones meaning; these lists quickly became popular poems. In 1870, the influential jewelers Tiffany & Co. published one of these poems, further popularizing the idea of a specific stone for each birth month.
Thanks to Wikipedia, we've included excerpts from this poem, below (in italics). Somewhere along the way, some of the birthstones meanings have changed and other special qualities became associated with each gem. For the month of June, even the stone changed! Pearl, alexandrite, and moonstone replaced the much more common agate, which comes in a bewildering number of varieties.
January - Garnet
They will ensure her constancy, True friendship, and fidelity.
Garnet comes from granatum which means seed - and they do resemble pomegranate seeds! They represent eternal friendship and trust.
February - Amethyst
Sincerity and peace of mind, Freedom from passion and from care, If they an amethyst will wear.
Amethyst comes from the Greek word for sober - amethystos. It was believed to keep the wearer clear-headed and quick-witted even while drinking.
March - Aquamarine or Bloodstone
In days of peril firm and brave, And wear a bloodstone to their grave.
The aquamarine's name comes from the Latin words for water (aqua) and the sea (marina) and was believed to provide sailors with a safe voyage - especially if the stone was engraved with Neptune's image.
Bloodstone is dark green flecked with blood-red and was once believed to heal blood diseases.
April - Diamond
Diamonds shall wear, lest bitter tears For vain repentance flow; this stone, Emblem of innocence, is known.
Diamonds are symbols of courage as well as everlasting love.
May - Emerald
And wears an emerald all her life Shall be a loved and happy wife.
Emeralds were being worn in Egypt over 2,000 years ago. Ancient Romans attributed it to Venus, the goddess of beauty and love. It's a symbol of rebirth, wisdom and patience.
June - Pearl, Alexandrite or Moonstone
With ring of agate on her hand Can health, wealth, and long life command.
Pearls symbolize purity and tears of joy.
The very rare Alexandrite was only discovered in Russia in 1831 during the reign of Czar Alexander II.
For centuries, moonstone's appearance was believed to change with the phases of the moon. It often shows either a star or cat's eye that appears to float over the surface of the stone.
July - Ruby
Then they'll be exempt and free From love's doubts and anxiety.
Rubies were once believed to be protection from evil. Now they're associated with love and passion.
August - Peridot or Sardonyx
The August-born without this stone [sardonyx], `Tis said, must live unloved and lone.
Peridot crystals are often found near volcanoes and were once believed to be the tears of Pele, the Hawaiian goddess of volcanoes.
Sardonyx was popular with ancient Greek and Roman soldiers. They often carried pieces engraved with images of heroes like Hercules; they believed the stones would help them be brave and achieve victory.
September - Sapphire
A sapphire on her brow should bind `Twill cure diseases of the mind.
During the Middle Ages, sapphires were believed to symbolize heaven and confer heavenly blessings on the wearer.
October - Opal or Tourmaline
But lay an opal on her breast, And hope will lull those woes to rest.
In Greek, Opallos means to see a change; opals are known for their flashes of brilliant, ever-changing color.
Tourmaline comes in a wide range of colors and sometimes even 2 or 3 colors in the same stone!
November - Topaz
Should prize the topaz's amber hue, Emblem of friends and lovers true.
For thousands of years, all yellow stones were prized as topaz, which represents love and affection.
Citrine is one of the yellow stones often mistaken for topaz. It's believed to share its warmth and energy with the wearer.
December - Turquoise, Zircon or Tanzanite
Place on your hand a turquoise blue; Success will bless whate'er you do.
Turquoise is French for Turkish stone and dates back to the thirteenth century. It's thought to be protection against harm.
Natural zircon is believed to relieve pain and provide restful sleep.
Tanzanite was discovered in Tanzania in the late 1960s and is still found only in that area.