December Birthstone
December Birthstone
Turquoise, tanzanite, zircon, and blue topaz — a month of blue and violet gems
December is unusual among the calendar months in being assigned four recognised birthstones by the major gemmological and trade authorities: turquoise, tanzanite, zircon, and blue topaz. A fifth stone, lapis lazuli, is recognised in some British and older European lists. The multiplicity reflects both the deep historical roots of turquoise as a talismanic gem and the deliberate modernisation of birthstone lists by trade bodies responding to consumer demand and market availability. All five stones share a broadly blue or blue-violet palette, giving December a coherent chromatic identity despite the diversity of its gems.
Turquoise — the Traditional Stone
Turquoise (CuAl₆(PO₄)₄(OH)₈·4H₂O) is the oldest and most historically embedded of December's birthstones. Its use as an ornamental material spans at least six millennia, documented in ancient Egyptian, Persian, pre-Columbian Mesoamerican, and Tibetan cultures. The finest material — opaque, even, and of a saturated sky-blue to blue-green hue — has historically come from the Nishapur district of Iran (formerly Persia), where the colour standard known as Persian turquoise was established. Significant deposits also occur in the American Southwest, particularly in Arizona (the Sleeping Beauty and Bisbee mines), New Mexico, and Nevada, as well as in China and the Sinai Peninsula.
Turquoise has a hardness of only 5 to 6 on the Mohs scale and is porous, making it susceptible to discolouration from cosmetics, oils, and prolonged exposure to light. The vast majority of commercial turquoise is treated: stabilisation with resins or polymers is standard practice and generally accepted in the trade when disclosed. Dyeing and the use of simulants such as howlite are also widespread, making laboratory identification important for fine specimens. Untreated, naturally coloured turquoise of Persian or high-grade American origin commands a significant premium.
Tanzanite — the Modern Addition
Tanzanite (the blue-violet gem variety of the mineral zoisite, Ca₂Al₃(SiO₄)₃(OH)) was added to the official American birthstone list by the American Gem Trade Association (AGTA) in 2002 — the first revision to the list since 1912. Found exclusively in a small mining area near Merelani in the Manyara Region of Tanzania, tanzanite is one of the most geographically restricted of all major gemstones. Its trichroic nature — displaying blue, violet, and burgundy depending on the viewing axis — and its strong pleochroism make it visually distinctive. Virtually all tanzanite in commerce has been heat-treated to suppress the brownish component and intensify the blue-violet saturation; this treatment is stable, universally accepted, and assumed unless a stone is specifically certified as untreated.
Tanzanite's inclusion as a December birthstone was commercially significant, providing a high-value alternative to turquoise at a time when the gem had achieved wide consumer recognition, partly through promotion by Tiffany & Co. following the stone's discovery in 1967. Its single-source origin raises long-term supply questions that distinguish it from most other birthstones.
Zircon — the Brilliant Underrated Gem
Blue zircon is perhaps the least well-known of December's birthstones despite possessing optical properties that rival those of diamond. Natural zircon (ZrSiO₄) has a refractive index of approximately 1.93–1.98 (high-type) and a dispersion of 0.039, producing a fire and brilliance that is immediately apparent in well-cut stones. Blue zircon is produced by heat-treating brownish or reddish zircon, primarily from Cambodia and Myanmar; the colour is generally stable under normal conditions but can fade with prolonged exposure to strong ultraviolet light.
Zircon suffers from persistent confusion with cubic zirconia, a synthetic simulant with which it shares no mineralogical relationship. This confusion has suppressed its market profile despite its genuine merits as a natural gemstone with a long history of use in jewellery. With a hardness of 7.5 and somewhat brittle facet edges due to its crystal structure, zircon requires moderate care in wear.
Blue Topaz — the Accessible Alternative
Blue topaz (Al₂SiO₄(F,OH)₂) is the most widely available and affordable of December's birthstones. Natural blue topaz is rare; virtually all blue topaz in the market has been irradiated and subsequently heat-treated to develop its colour. Three commercial grades are recognised: Sky Blue (pale), Swiss Blue (medium, vivid), and London Blue (deep, slightly inky). These treatments are permanent and stable, and are universally disclosed as standard practice. Brazil is the dominant source of topaz rough used for blue treatment, with Nigeria and Sri Lanka also contributing.
With a hardness of 8 and good clarity, blue topaz is durable and well-suited to everyday jewellery. Its affordability relative to tanzanite and fine turquoise makes it the most commercially prominent December birthstone by volume, though it lacks the rarity or historical depth of its counterparts.
Lapis Lazuli — the British Tradition
Lapis lazuli, the deep-blue metamorphic rock composed principally of the mineral lazurite with pyrite and calcite inclusions, appears on some British and older European birthstone lists for December. Its historical prestige — it was ground into ultramarine pigment and used in Renaissance painting, and was mined in Afghanistan's Badakhshan province for over six thousand years — gives it a cultural weight that rivals turquoise. It does not appear on the current AGTA or Jewelers of America lists, and its status as a December stone is therefore a matter of regional tradition rather than modern standardisation.
Choosing Among December's Stones
The range of December birthstones offers a breadth of price points, durabilities, and aesthetics uncommon in a single birth month. Turquoise suits those drawn to historical depth and an opaque, earthy aesthetic; tanzanite appeals to those seeking a fine, transparent coloured gem of rarity and intensity; zircon rewards those who value optical brilliance in a natural stone; and blue topaz provides accessible, durable everyday wear. All share the blue and violet tones that define the month's chromatic character.