May has emerald on both lists, and the traditional United Kingdom list adds chrysoprase, an apple-green chalcedony.
The two lists for May
Modern list (American Gem Society): Emerald
Traditional list (United Kingdom, National Association of Goldsmiths, 1937): Emerald, Chrysoprase
There is no single official birthstone list, so you are free to choose whichever you prefer. You can check any month with our birthstone finder.
About the stones
Emerald
Emerald is green beryl, coloured by traces of chromium or vanadium, with a hardness of 7.5 to 8 but often internal fractures that make it brittle. Colombia, Zambia and Brazil are leading sources, and ancient mines near the Red Sea in Egypt are traditionally called Cleopatra’s mines. Most emeralds are treated with oil or resin to fill fractures, which is accepted in the trade if disclosed.
Chrysoprase
Chrysoprase is apple-green chalcedony, coloured by nickel, with a hardness of 6.5 to 7. It is a microcrystalline quartz, found in Australia among other places, and it is cut into cabochons and beads.
How the idea developed
Emerald is among the oldest gems in any birthstone tradition, and it has been prized in Egypt, India and South America for thousands of years. The 1912 American list and the 1937 United Kingdom list both keep emerald, but only the British list keeps chrysoprase. The link between the twelve stones of the breastplate and the months, traced to Josephus, is a tradition, and scholars are not certain which modern stone each Hebrew name in the passage describes.
For the wider history of the idea, see our birthstones overview. The modern list follows the American Gem Society, and the traditional list follows the 1937 United Kingdom list, both summarised in Wikipedia, Birthstone.
A different system
Vedic gemstone (ratna) guidance is a different system from birthstones. It is tied to the nine planets and to a birth chart, not to the calendar month, and it belongs to the Vedic tradition, which is covered on our sister site, Nidarshana Vedh. Nothing on this page is Vedic advice.
A note on health. Crystals are valued for beauty, meaning and ritual. There is no scientific evidence that they treat health conditions. If you are struggling with your health or mood, please speak to a doctor.
Frequently asked questions
↳ Is it normal for emeralds to have flaws?
Yes. Most natural emeralds have internal fractures, sometimes called the garden, and many are filled with oil or resin. Ask whether a stone has been treated.
↳ What is chrysoprase?
It is apple-green chalcedony coloured by nickel, a type of microcrystalline quartz. It is softer and more opaque than emerald.