Ask for your birthstone in a jewellery shop and you will be given a stone from a list drawn up by American jewellers in 1912 and revised a handful of times since. It is a good list, and it has settled into tradition, but it is a commercial one, and it sits on top of two much older systems: the twelve stones of the zodiac and the nine planetary gems of the subcontinent. This article gives the modern list month by month and notes where the older traditions agree or differ.
January - Garnet
Deep red garnet, for constancy and protection on journeys. It suits Capricorn, which covers most of January, and Aquarius, which begins on the twentieth.
February - Amethyst
Purple quartz, for clarity and sobriety. The stone of Aquarius, and in Vedic practice the gentle stand-in for blue sapphire, which makes it the right stone for both signs Saturn rules.
March - Aquamarine (and Bloodstone)
Sea-blue beryl for calm and courage; bloodstone is the older alternative. Pisces takes aquamarine naturally; Aries, which begins around the twenty-first, is a Mars sign and has more in common with bloodstone.
April - Diamond
The hardest stone, for strength and enduring love. It is Venus's gem, which suits Taurus at the end of the month; for Aries in the first three weeks it is worn more for its brilliance than for any planetary reason. White sapphire and opal are the accepted substitutes.
May - Emerald
Green beryl, for wisdom and growth. Mercury's stone, so it suits Gemini from the twenty-first, and it is the traditional stone of Taurus as well, which makes May one of the months where the modern and older lists agree entirely.
June - Pearl (also Moonstone and Alexandrite)
The Moon's gem, for peace of mind and the emotions. Cancer, the Moon's own sign, begins around the twenty-first, and moonstone, the second June stone, is the Moon's other gem. Alexandrite, the colour-changing stone, was added later and is rare enough that most June wearers never see one.
July - Ruby
The Sun's gem, for vitality and authority. Leo begins around the twenty-third, so July's stone matches its second sign; for Cancer in the first three weeks the tradition would reach for pearl instead.
August - Peridot (also Spinel and Sardonyx)
The green stone of renewal, and one that Pakistan produces better than almost anywhere. Leo and Virgo share August; peridot is read as a gentle Mercury stone and suits Virgo well, while Leo would traditionally look to ruby or amber. Spinel was added in 2016; sardonyx is the oldest August stone.
September - Sapphire
Blue corundum, Saturn's gem. This is the month where modern and astrological lists diverge most: Virgo and Libra, September's signs, are Mercury and Venus signs, and the astrologer would suggest emerald or opal before Neelam. The modern list chose sapphire for its history as a stone of wisdom and its September colour.
October - Opal (and Tourmaline)
Venus's stones, for harmony and beauty. Libra, the Venus sign, covers most of October, and Scorpio, from the twenty-third, has its own affinity with the deeper tourmalines. This month the lists agree.
November - Topaz (and Citrine)
Golden stones for warmth and Jupiter's blessings. Scorpio takes topaz on this site; Sagittarius, from the twenty-second, is Jupiter's sign and takes citrine or yellow sapphire as the natural extension.
December - Turquoise (also Tanzanite and Zircon)
Feroza, the traveller's stone and the stone of Sagittarius, with tanzanite added in 2002 and blue zircon as the older alternative. Capricorn, from the twenty-second, would traditionally look to garnet, which conveniently is January's stone.
Which list should you follow?
If you want a stone as a gift or an ornament, the month list is a fine guide and no one will argue with it. If you want a stone to wear for a reason, an astrological one, use the zodiac sign rather than the month, because the months straddle two signs, and then check the planetary system if you can get a chart read, because it looks at more than the Sun. The gemstone pages on this site give all three where they differ, and the article on how to wear a gemstone explains what to do once you have chosen.