Tanzanite

Tanzanite is worth being plain about, because its story cuts against the usual romance of birthstones as ancient or timeless. It’s the blue-to-violet gem variety of the mineral zoisite, and it was only identified as a distinct gemstone in 1967, discovered in the Merelani Hills near Mount Kilimanjaro in Tanzania — the country it takes its name from, and, as far as is currently documented, still the only commercial source of gem-quality material anywhere in the world. That single-locality status is genuinely unusual; most major gemstones are found across multiple countries, but tanzanite’s known supply is tied to one region.

Its birthstone status is equally recent and equally worth stating directly: tanzanite was added to the official December birthstone list only in 2002, when the American Gem Trade Association included it alongside the long-established turquoise (and, later, zircon). That’s not a correction of an old oversight — it’s a 21st-century trade decision made 35 years after the stone itself was discovered and roughly ninety years after the modern US monthly list was first set in 1912. A site built around telling the truth about where birthstone claims come from should say this plainly rather than let tanzanite sit quietly among far older December traditions as if it had always been there.

Tanzanite’s color, ranging from blue through violet-purple, often shows pleochroism — different colors depending on the viewing angle through the crystal — which heat treatment (a standard, widely-disclosed practice in the trade) typically enhances and stabilizes in commercial material. Gemologically it’s a distinct species (zoisite), not a variety of any other blue stone, despite occasionally being confused with sapphire by non-specialists on sight.

Care matters more for tanzanite than for many gems on this site. At Mohs 6 to 7, it’s softer than quartz, topaz, or corundum, and it also has pronounced cleavage, meaning a hard knock — not just a scratch — can crack or chip it. That combination makes it a better fit for earrings and pendants than for a ring in daily hard use, unless the setting protects the stone well. On the calendar, it’s a December stone on the UK 2013 list (paired with turquoise) and the current US list (with turquoise and zircon); it doesn’t appear on the historical or US 1912 December lists, both of which predate its discovery. It’s also the modern jewelry-trade zodiac stone for Sagittarius — the one sign where the two zodiac systems tracked here actually agree, since the older tropical zodiac independently arrives at topaz for the same sign via the historical December entry.