December has more birthstones than any other month — turquoise, zircon and tanzanite on the modern list, plus blue topaz, which is not officially on it and is what most shops will actually sell you. Four stones, all blue, all completely different materials.
One of them was unknown to science until 1967. One of them is routinely confused with a lab-made imitation that is not related to it at all. And one of them is blue only because somebody irradiated it.
Here is which is which.
Tanzanite: younger than the moon landing
In 1967, in the Merelani Hills of northern Tanzania, blue crystals were found that turned out to be a variety of the mineral zoisite nobody had seen in that colour before. Tiffany & Co. took it on the following year, decided blue zoisite sounded too much like suicide, and named it tanzanite after the country.
The remarkable part is not the age. It is the geography. Commercially significant tanzanite comes from a strip of hills roughly seven kilometres long, and nowhere else on earth. Every tanzanite in every shop window in the world came out of that one small area. Geologists put the odds of the specific conditions recurring at something on the order of a million to one.
Nearly all of it is heated. Rough tanzanite is usually brownish, and gentle heat removes that and brings the blue-violet forward. This is standard, stable, permanent, and disclosed by anyone reputable. It is also worth knowing that tanzanite is pleochroic to an unusual degree: the same crystal shows blue, violet and burgundy along different axes, and the cutter chooses which one you get to see.
It is a soft stone for daily wear — 6 to 7 on the Mohs scale, with definite cleavage. It is happier as a pendant than as a ring you garden in.
Zircon is not cubic zirconia
This is the single most common mix-up in the birthstone list, and the names are entirely to blame.
Zircon is a natural mineral — zirconium silicate — that has been mined and worn for thousands of years. Cubic zirconia is a laboratory-made diamond simulant, invented in the 1970s, a different compound with a different structure. They share four letters and nothing else.
Natural zircon is, in fact, one of the more remarkable materials on the planet. Zircon crystals from the Jack Hills in Western Australia have been dated to about 4.4 billion years — the oldest known fragments of the earth's crust, older than any rock. They survive because zircon is chemically stubborn: it locks in uranium as it grows and excludes lead, which is precisely what makes it the most reliable clock geologists have.
As a gem, zircon has high dispersion — it splits white light into colour more strongly than almost anything except diamond. Before diamond simulants existed, colourless zircon was what you used. Most blue zircon on the market is brown zircon that has been heated.
Blue topaz: honest about where the blue came from
Blue topaz is on almost every December list in the trade, and it is the cheapest of the four by a wide margin. There is a reason for that.
Natural blue topaz exists and is very pale and very rare. Effectively all blue topaz sold — Sky Blue, Swiss Blue, London Blue — starts as colourless topaz, is irradiated to create colour centres in the crystal, then heated to stabilise the shade. The three trade names are three points on that scale, from palest to deepest.
The treatment is permanent, the material is genuine topaz, and treated stones are held until residual radioactivity falls below regulatory limits before they can be sold. None of that is a problem. What would be a problem is not telling you, so: our blue topaz is treated, like all of it.
Turquoise: the one that was carried
The oldest of the four by a distance — six thousand years of being mined, traded and worn, from Egypt to Persia to the American Southwest. It is also the only one of the four that is a copper mineral, which is where the colour comes from: copper for blue, iron pulling it towards green.
Turquoise is porous, and much of what is sold is stabilised with resin to stop it absorbing oil and changing colour on the wearer. We have written about it separately: Turquoise: six thousand years of being carried →
What we make for December
Our December stone is topaz, and it comes in two depths:
- Blue Antler — Swiss blue topaz, the deeper shade, as a necklace and ring set
- Plain Speaking — sky blue topaz, the pale one, in the plainest setting we make
- Open Current — sky blue topaz with amethyst, as drop earrings
The everyday version is Month in Colour: one baguette stone on a fine chain, solid 925 sterling silver plated in gold rather than plated brass. The December stone is blue. It is cubic zirconia cut to the birthstone colour, not a natural gemstone — which, given everything above, is a distinction worth being precise about. It is on the product page, not in the small print.
Made after you order, so allow up to ten days.
All twelve months, and what each stone actually is →
Where stone meanings actually come from →
A birthstone is a date, not a diagnosis. Nothing here treats, prevents or cures anything.