Where birthstones come from
The idea traces back to the twelve gems set in the breastplate of the high priest in Exodus. Linking those stones to calendar months came much later, in eighteenth-century Poland, and the list most people use today was adopted in 1912 by the National Association of Jewelers at its Kansas convention.
The list keeps changing
A 1952 revision added alexandrite to June, pink tourmaline to October and citrine to November, and swapped December's lapis lazuli for zircon. The American Gem Trade Association added tanzanite to December in 2002, and spinel joined August in 2016. That is why charts published in different decades disagree — none of them is wrong, they simply reflect different revisions.
National lists differ
Britain's National Association of Goldsmiths published its own standard in 1937, and Japan's jewellery association added ten stones in 2021. There is no international body governing birthstones; each market follows its own trade custom.
Choosing a stone that lasts
Check Mohs hardness before buying a ring. Diamond, ruby, sapphire and topaz hold up to daily wear. Emerald rates highly on hardness but its inclusions make it prone to chipping, and opal and pearl are soft enough that pendants and earrings are the safer setting.