Why two lists?
Birthstone assignments were never universal. Historical sources drew from the 12 stones of the High Priest's breastplate (Exodus 28), Arabic and Persian lapidaries, Hindu Navaratna (nine-gem) tradition, and various European folk traditions. The 1912 US list was a commercial consolidation: jewelers agreed on one stone per month to simplify marketing.
Subsequent updates responded to supply concerns (October's tourmaline replaced opal as “not bringing bad luck” to gift givers) and marketing pressure (tanzanite December was driven by Tanzanite One's promotional effort).
Month-by-month details
January
Garnet has been January's stone in both traditional and modern lists, stable across centuries. The January garnet is typically pyrope or almandine (deep red); rhodolite and other garnet varieties are acceptable substitutions.
February
Amethyst is the modern stone. The traditional list sometimes includes bloodstone, carried over from older sources. Bloodstone is still occasionally used as an alternate.
March
Aquamarine is the modern stone, added to the 1912 list. The traditional stone is bloodstone. Many modern vendors offer both, with bloodstone as the “traditional” option.
April
Diamond is the modern and traditional stone. Historical European sources occasionally listed sapphire for April; the diamond association is older than commercial standardization but cemented by 20th-century engagement-ring marketing.
May
Emerald is the modern and traditional stone. Older sources sometimes included agate for May; emerald has been dominant since at least the medieval period.
June
Pearl is the traditional June stone. Moonstone was added in 1912. Alexandrite was added in 1952 following the mineral's commercial rise. All three are currently listed as modern options.
July
Ruby is modern and traditional. Some older Middle Eastern sources used carnelian for summer months; ruby has dominated since Greco-Roman lapidary tradition.
August
Peridot is the modern stone, added in 1912. Sardonyx was the traditional stone. Spinel was added in 2016 following industry effort to recognize the gem after long conflation with ruby.
September
Sapphire is modern and traditional. Lapis lazuli appears in some older sources as an alternate for September; sapphire has dominated for over 500 years.
October
Opal is the traditional stone. Tourmaline was added in 1912 after concern that opal carried bad-luck associations in some folk traditions. Both are currently listed as modern options.
November
Topaz is traditional. Citrine was added to the modern list in 1952 as a more affordable alternative (at the time, imperial topaz was considerably more expensive than citrine).
December
Turquoise is traditional. Blue zircon was added in 1912. Tanzanite was added in 2002 after commercial pressure from tanzanite industry groups. All three are modern options.
Which to give?
A note on historical birthstones
Ancient and medieval birthstone assignments were chaotic. The same stone could be listed for different months in different sources, and Hebrew, Arabic, Italian, and Indian traditions each had their own lists. Modern consolidation into one list per month is a 20th-century commercial simplification, not a historical truth.
This is worth knowing if someone insists their stone is the “real” birthstone for a given month: they may be drawing on one tradition among many.