December Birthstone

December closes out the birthstone calendar with three modern options — turquoise, tanzanite, and zircon — plus turquoise carrying forward as the traditional stone. It's a fitting range for the year's final month: one of the oldest gem materials humans have used, one of the newest gem discoveries in modern history, and one whose individual crystals can be older than almost anything else on Earth.

Turquoise: Among the Oldest Gems in Human Use

Turquoise is an opaque, blue-to-green copper aluminum phosphate mineral, and it has one of the longest documented histories of ornamental use of any gemstone — turquoise jewelry has been found in Egyptian tombs dating back thousands of years, and it holds deep cultural and spiritual significance in Persian tradition as well as among numerous Native American nations of the American Southwest, particularly the Navajo and Zuni, whose silver-and-turquoise jewelry traditions, developed from the mid-19th century onward after silversmithing techniques were learned and adapted, remain internationally recognized art forms today. The English name, somewhat oddly, comes from the French pierre turquoise, meaning 'Turkish stone' — not because the stone was mined in Turkey, but because early European supplies reached the continent via Turkish trade routes from mines actually located in Persia (modern-day Iran).

Tanzanite: A Gem Found in One Place on Earth

Tanzanite is a blue-to-violet variety of the mineral zoisite, and it holds a genuinely rare distinction: it has only ever been found in one small area on Earth, the Merelani Hills near Mount Kilimanjaro in Tanzania — a single-source origin shared by very few other gemstones. It was discovered in 1967, making it one of the most recently identified gem materials to reach mainstream jewelry, and it was named and marketed by Tiffany & Co. shortly after its discovery, who chose 'tanzanite' specifically because 'blue zoisite' — the more technically direct name — was judged, not without reason, to be an unfortunate mouthful for retail jewelry counters. Most tanzanite on the market has been heat-treated to enhance its blue-violet color from the material's more common natural brownish tone.

Zircon: An Ancient Mineral, Not a Synthetic

Zircon is frequently and mistakenly confused with cubic zirconia, a lab-created diamond simulant — but zircon is an entirely different, naturally occurring mineral (zirconium silicate) with a long history of its own. In fact, zircon holds a genuinely remarkable scientific distinction: some zircon crystals found in Australia have been dated at over 4 billion years old, making them among the oldest known minerals on Earth and a significant tool for geologists studying the planet's earliest history. Gem-quality zircon occurs in multiple colors, but the blue zircon most commonly seen in jewelry is typically produced by heat-treating naturally brown zircon, similar to the treatment process used for tanzanite and much of the commercial blue topaz market.

Care Across the Trio

Turquoise, at only 5–6 on the Mohs scale and naturally porous, is sensitive to oils, cosmetics, and even prolonged sun exposure, and should be cleaned only with a soft dry cloth. Tanzanite, at 6–7 Mohs with distinct cleavage, needs careful handling to avoid chipping. Zircon, at 6–7.5 Mohs, is more durable but can show wear along facet edges over time with regular use.

American Southwest Turquoise and Stabilization

High-quality American turquoise from historic Southwestern mines — including the Sleeping Beauty mine in Arizona and several New Mexico deposits associated with Native American silverwork — has become increasingly scarce as many of these mines have closed or been depleted over recent decades, driving up prices for documented American-mined material relative to more abundant Chinese and other international turquoise sources. Because natural turquoise is often soft and porous, a large share of commercial turquoise today is 'stabilized' — treated with a clear polymer resin that fills microscopic pores, hardening the stone and making its color more stable and consistent. Stabilization is standard, disclosed practice, but untreated, natural-grade turquoise from historic mines commands a significant premium among collectors specifically because it wasn't chemically altered.

Tanzanite's Trichroism: Three Colors in One Stone

Tanzanite displays a genuinely unusual optical property called trichroism, meaning the stone shows different colors — typically blue, violet, and a brownish-red — depending on the angle from which it's viewed relative to its crystal structure, before cutting and heat treatment settle it into the more uniform blue-violet appearance seen in finished jewelry. Skilled cutters orient the rough crystal specifically to maximize the desired blue-violet face in the finished gem, meaning the cutting choices made on a single tanzanite crystal directly determine which of its three natural colors ends up dominating what a wearer actually sees.

Ancient Zircon and What It Tells Geologists

Beyond gem use, the ancient zircon crystals mentioned above — some dated to over 4 billion years old, found in the Jack Hills region of Western Australia — are geologically significant well beyond their age alone: trace oxygen isotopes preserved within these zircons have given geologists evidence suggesting liquid water may have existed on Earth's surface far earlier in the planet's history than previously assumed, pushing back scientific estimates for when Earth's surface first cooled enough to support oceans. It's a striking example of a birthstone mineral carrying genuine, actively studied scientific significance entirely separate from anything related to jewelry or December birthdays.

Turquoise in Native American Jewelry Traditions Specifically

Navajo and Zuni silverwork traditions, though both centered on turquoise-and-silver combinations, developed distinct stylistic approaches worth distinguishing: Navajo work has historically favored larger, boldly set single turquoise stones in relatively simple silver settings, while Zuni artisans became particularly known for intricate inlay and 'petit point' techniques using many small, precisely cut turquoise pieces set closely together in detailed patterns, sometimes combined with coral, jet, and shell in a style called mosaic inlay. Both traditions trace back to the mid-to-late 19th century, after Navajo smiths learned silversmithing techniques from Mexican metalworkers and adapted them using local turquoise, a craft history that's now recognized and protected under U.S. federal law regarding the marketing of goods as genuine Native American-made art.

Tanzanite's Rapid Rise From Unknown to Iconic

Tanzanite's journey from complete obscurity to global recognition happened remarkably fast even by gem-industry standards: discovered in 1967, marketed by Tiffany & Co. beginning in 1968, tanzanite became one of the most requested colored gemstones in American jewelry within roughly a decade, an unusually rapid rise for a stone with no prior historical reputation, mythology, or established royal provenance to draw on — its appeal rested almost entirely on the color itself and Tiffany's marketing reach, a genuinely modern case study in how a brand-new gem material can be built into a major category in a short span of time, something that took centuries for stones like ruby or emerald to achieve organically.