December Birthstone
Turquoise, tanzanite, and zircon — December's trio offers deep blue through sky blue for the winter month.
December has three modern birthstones: turquoise (the oldest traditional stone), tanzanite (added 2002), and blue zircon (added in the 20th century). All three are blue, matching the visual colour of winter sky, frost, and northern light. Turquoise is a copper aluminium phosphate mineral (CuAl₆(PO₄)₄(OH)₈·4H₂O) with an opaque to waxy blue-to-green colour used since at least 5000 BCE in Egyptian jewellery. Tanzanite is a blue-violet variety of zoisite (Ca₂Al₃(SiO₄)₃(OH)) found only in a single deposit near Arusha, Tanzania. Blue zircon (ZrSiO₄) is a natural mineral frequently heat-treated to intensify blue colour. December's blue trio spans the ancient opaque blue of turquoise, the trichroic gemological rarity of tanzanite, and the high-dispersion fire of blue zircon.
Quick facts
- Item type
- Birthstone month
- Color range
- sky blue to blue-green (turquoise), blue-violet to purple (tanzanite), light to medium blue (zircon)
- Birthstone month
- December (traditional)
Turquoise: The Oldest Blue Gem
Turquoise is one of the first gems used by humans for jewellery and decoration. Egyptian use of turquoise dates to approximately 5000–4000 BCE; Tutankhamun's burial mask contained turquoise inlays. Native American turquoise traditions in the American Southwest (Navajo, Pueblo, Zuni) have used turquoise from Cerrillos (New Mexico) and other local deposits for centuries in ceremonial and personal jewellery. Turquoise is hydrated copper aluminium phosphate, coloured sky blue to blue-green by copper (Cu²⁺) — more copper produces a purer blue; more iron shifts the colour toward green. It is typically opaque, waxy to subwaxy in lustre, and relatively soft at Mohs 5–6. The finest turquoise comes from Iran (Nishapur region) — Persian turquoise — described as vivid, pure blue with minimal matrix (the dark veins of host rock sometimes prized in American turquoise but considered impurities in Persian standards). Arizona (Sleeping Beauty mine, now closed) produced fine sky-blue turquoise. Most commercial turquoise is treated with stabilising resin to harden and improve colour.
Tanzanite: The Single-Source Gem
Tanzanite is found commercially in only one location on Earth: the Merelani Hills near Arusha in northern Tanzania, on the slope of Mount Kilimanjaro. Tiffany & Co. named the stone 'tanzanite' and launched it commercially in 1968 (discovery approximately 1967). The stone is a variety of zoisite coloured by vanadium (V³⁺), producing a distinctive blue-violet to violet-blue colour with strong trichroism — viewed along different crystal axes, tanzanite appears blue, purple, and red-brown. Most tanzanite is heat-treated at approximately 600°C to remove the brownish-yellow component and enhance the blue-violet; treated and untreated tanzanite are essentially indistinguishable. As a single-source gem, tanzanite's availability is geologically limited to a 14 km² mining zone — its supply will eventually exhaust. This finite nature is a central marketing element. Tanzanite rates 6.5 Mohs and has perfect cleavage, making it relatively fragile for jewellery use compared to sapphire.
Zodiac Connection: Sagittarius
December birthstones align with Sagittarius (the archer), governing approximately November 22 to December 21. Sagittarius is a fire sign associated with exploration, philosophy, freedom, and the pursuit of higher truth — the archer aims toward distant horizons. December's trio of blue gems resonates with Sagittarius's expansive character: the blue of sky and distant landscape, the limitless horizon that the archer scans. Turquoise's ancient connection to travel protection (Tibetan, Native American, and Persian traditions all used turquoise as a protective amulet for journeys) aligns with Sagittarius's adventurous spirit. Tanzanite's trichroism — its ability to show blue, violet, and burgundy simultaneously depending on viewing angle — mirrors Sagittarius's capacity to hold multiple philosophical perspectives. Blue zircon's exceptional brilliance (highest RI of any natural gem except diamond) suggests the brightness of winter stars, under which the archer navigates.
Sources & further reading (3)
- gemological-institute — accessed 2026-05-08
- gemological-institute — accessed 2026-05-08
- encyclopedia — accessed 2026-05-08
Frequently asked questions
Is tanzanite a good investment gem?
Tanzanite has been marketed partly on its single-source scarcity, and prices have fluctuated significantly since its commercial introduction in 1968. The supply argument — that mining a 14 km² zone is finite — is valid in geological terms, but the timeline for exhaustion is uncertain and mines have repeatedly extended their productive lives with new discoveries at depth. Tanzanite prices peaked in the early 2000s, declined after the 2008 financial crisis, and have recovered variably since. As a PM applying investment criteria to any gem: gem investment generally requires deep market knowledge, authenticated specimens with laboratory certificates, and long holding horizons. Tanzanite lacks the established auction records of ruby, sapphire, or emerald of equivalent quality, making price benchmarking difficult. It is an excellent gem for personal enjoyment; its investment credentials are not comparable to the three classic precious stones. GIA issues grading reports for tanzanite identifying colour, clarity, and treatment.
What is blue zircon and why is it different from cubic zirconia?
Zircon (ZrSiO₄) is a natural mineral — one of the oldest minerals on Earth, with some Australian zircons dated to 4.4 billion years ago. Blue zircon is produced by heat treating brownish to reddish zircon from Indochina (Cambodia, Myanmar) at high temperatures, which creates the blue colour through reduction of iron. Zircon has the highest refractive index of any natural gem (RI 1.925–1.984 for high zircon) and strong dispersion (0.038), giving it exceptional fire and brilliance. It is also significantly radioactive in some specimens (due to uranium and thorium substitution in the crystal structure), which over time damages the crystal lattice (metamictisation) — this is why many commercial zircons are 'high' zircon (undamaged) or 'intermediate' (partially damaged), with different optical properties. Cubic zirconia (CZ) is a synthetic material (ZrO₂ stabilised by yttrium) — not related to natural zircon. The naming similarity causes widespread confusion; they are entirely different substances.
Why does turquoise turn green over time?
Turquoise can change colour from blue toward green over time due to several mechanisms. Chemical changes: the copper-aluminium phosphate structure can absorb oils, cosmetics, skin chemistry, and environmental chemicals that alter the colour centres or introduce new chromophores. Most commonly, absorption of oils into untreated turquoise causes a darkening and greening of the colour. Persian 'robin's egg' blue turquoise is particularly susceptible to colour change from oil absorption. Dehydration: turquoise contains water (H₂O) in its structure; loss of this structural water through heat or very dry conditions can cause colour shifts toward green or grey. Stabilised turquoise (resin or wax-impregnated) is much less susceptible to colour change because the pores are filled. Untreated natural turquoise requires protective care: avoid contact with oils, cosmetics, household chemicals, and prolonged heat. Turquoise should never be cleaned ultrasonically or with steam.