Birthstone.

A birthstone is a gemstone linked to the month a person was born in. The custom is old and layered. Early sources connected the twelve stones of the breastplate of the high priest in the Book of Exodus first to the twelve tribes of Israel, then (by writers such as Josephus in the first century and St Jerome in the fourth) to the twelve signs of the zodiac, and only later to the twelve months of the year. The shift from sign to month seems to have settled in central Europe around the eighteenth century, where wearing one stone for each month, rather than owning all twelve, became the more practical fashion.

The modern list

The month-by-month version most westerners know today was formalised in 1912 by what was then the American National Association of Jewelers (later Jewelers of America), with later additions by the Jewelry Industry Council of America in 1952 and further updates by the American Gem Trade Association as recently as 2002 (when tanzanite was added to December) and 2016 (when spinel joined ruby for August). The standard modern list runs: garnet for January, amethyst for February, aquamarine or bloodstone for March, diamond for April, emerald for May, pearl, moonstone, or alexandrite for June, ruby for July, peridot, spinel, or sardonyx for August, sapphire for September, opal or tourmaline for October, citrine or topaz for November, and turquoise, tanzanite, or zircon for December.

Older traditions retain their own lists. The fifteenth-century European list places hyacinth in January and bloodstone in March. The Hindu navaratna assigns nine stones to nine planets rather than twelve months. Zodiac birthstones, assigned by sign rather than calendar month, overlap with the modern list but do not match it exactly: an Aries born in late March is given bloodstone or diamond in some sources, aquamarine or jasper in others.

In practice

Birthstones make easy gifts because the link is personal without being presumptuous. They also give beginners a natural place to start a small collection. The trade additions are worth knowing about. Pearl was added to June partly because cultured pearls were becoming affordable in the 1910s. Tanzanite joined December in 2002, fewer than fifty years after the stone was first identified in Tanzania in 1967. Citrine sits on November's list largely because most commercial citrine is heat-treated amethyst, which let jewellers offer a yellow stone at a friendlier price than topaz.

A common misconception is that the birthstone list is ancient and fixed. It is neither. Most western birthstone lists are products of the twentieth-century jewellery trade, shaped by what stones were available, fashionable, and profitable to sell. This does not make them empty. A list that has been used for a hundred years carries its own weight of association, and a stone given because it lines up with a birth month tends to become meaningful through wearing.

When buying a birthstone, it helps to ask which list is being used and why. A March piece offered as bloodstone is drawing on the older European tradition; the same month offered as aquamarine is drawing on the 1912 list. Neither is more authentic. They are answers to slightly different questions about what a birthstone is for. The most useful thing a buyer can do is pick the stone that draws the eye, then check which list supports it. The custom rewards small acts of attention more than perfect orthodoxy.