December Birthstone: A Buyer's Guide to Tanzanite, Zircon and Turquoise

December is the only month with three birthstones that are all, in their own way, blue. Tanzanite, zircon and turquoise could hardly be more different gems: one is a velvety violet-blue that did not exist in the trade before 1967, one is among the oldest minerals on Earth and throws more fire than almost any natural gem, and one is an opaque sky-blue stone that has been carved and worn for thousands of years. If you were born in December, or you are buying for someone who was, this guide walks through what each stone really is, how it is treated, how it wears, and how to choose well.

Why December Has Three Birthstones

The modern birthstone list was standardized in 1912 by the association that became Jewelers of America, and December's original stone was turquoise. Zircon was added to the list in 1952, and tanzanite joined in 2002 when the American Gem Trade Association made the first change to the list in half a century. Some British lists also include blue topaz for December, which is why you will occasionally see four stones quoted. All of them sit within the blue family, but that is roughly where the resemblance ends, so it helps to think of them as three separate purchases with three separate sets of trade-offs. If you would like a broader map of the gem world first, our gemstone types page is a good place to start.

Tanzanite: The Newest December Birthstone

Tanzanite is the blue-to-violet variety of the mineral zoisite, colored by vanadium. Gem-quality material has only ever been found in one place: a few square kilometers of the Merelani Hills near Arusha in northern Tanzania, close to Kilimanjaro. Maasai herders are credited with the 1967 find, Tiffany & Co. named the stone after the country and launched it in 1968, and within a generation it went from unknown to one of the most recognized colored gems in the world. A single-source gem with a mine that will not last forever is a large part of tanzanite's appeal, and a large part of the reason serious buyers are paying attention now.

Tanzanite color and pleochroism

What makes tanzanite look the way it does is pleochroism, the property of showing different colors along different crystal directions. Tanzanite is strongly trichroic: look down one axis and it reads blue, another violet, and the third a burgundy-brown. The cutter's job is to orient the rough so the most desirable color faces up through the table, and that decision has real consequences. Orienting for the purest blue usually costs more of the rough than orienting for violet, which is why a genuinely blue-dominant tanzanite of the same weight is scarcer and more valuable than a violet one.

This is also why tanzanite seems to change personality with the lighting. Under daylight or cool LED it leans blue; under warm incandescent light the violet and even reddish flashes come forward. It is not a treatment, a flaw, or a trick of the setting. It is the same stone showing you a different axis. The finest stones are a vivid, saturated violetish blue with medium to medium-dark tone; paler, lighter material is pretty and much more affordable, but it does not have the depth that makes people stop and look. For more on how tone and saturation drive value across every species, see our guide to gemstone color.

Is tanzanite heat treated?

Almost always, and you should assume so unless a lab report says otherwise. Straight from the ground most zoisite from Merelani is brownish, because that burgundy-brown pleochroic direction dominates. Gentle heating, typically in the region of 600°C, removes the brown component permanently and leaves the stone dichroic in blue and violet. This is a mild, stable treatment that adds nothing to the stone and takes nothing away once done, and the entire market prices tanzanite on the assumption it has been heated. Unheated tanzanite that is naturally blue does exist, is very rare, and commands a premium when a laboratory can confirm it. Our article on gemstone treatments explains where heat sits on the spectrum from routine to problematic.

Tanzanite durability and care

Here is the honest part. Tanzanite sits around 6 to 7 on the Mohs scale, which is softer than sapphire, and more importantly it has one direction of perfect cleavage. A sharp knock at the wrong angle can split it. That makes tanzanite ideal for pendants, earrings and brooches, and a considered choice for rings. If you want a tanzanite ring, and many people rightly do, choose a bezel or a protective halo, avoid claw-only settings with exposed corners, and take it off for the gym, the garden and the washing-up. Clean it with warm soapy water and a soft brush only; ultrasonic and steam cleaners are not safe for tanzanite. We cover the difference between hardness, toughness and stability in our gemstone durability guide.

Tanzanite is a stone we hold in depth at Sapphire Row. You can browse current loose tanzanites or read more on our tanzanite page.

Zircon: The Oldest Gem With the Most Fire

First, the confusion that follows zircon everywhere: zircon is not cubic zirconia. Zircon is a natural mineral, zirconium silicate, that forms in the Earth's crust and has done so for a very long time; tiny zircon crystals from Western Australia have been dated to around 4.4 billion years, making them the oldest known material on the planet. Cubic zirconia is a laboratory-grown zirconium oxide invented in the 1970s as a diamond imitation. They share a few letters and nothing else.

Why zircon sparkles the way it does

Zircon has an unusually high refractive index, roughly 1.92 to 1.98 for the common high type, and a dispersion of about 0.039, only a little below diamond's 0.044. In plain terms, it bends light hard and splits it into rainbow flashes. A well-cut zircon in a lively color has a sparkle that most colored stones simply cannot match, which is why colorless zircon was sold as a diamond substitute for centuries before cubic zirconia existed. Read our piece on cut quality to understand why a good cutter matters so much more in a high-dispersion material like this.

Zircon also has strong birefringence, which gives a gemologist an easy identification test: look through the table with a loupe and the back facet edges appear doubled. Cubic zirconia, glass and diamond all show single edges. It is one of the quickest ways to confirm you are looking at the real mineral.

Zircon colors, and where blue zircon comes from

The December birthstone is usually shown as blue zircon, but the mineral occurs naturally in a much wider palette: cognac and honey browns, golden yellows, oranges, reddish "hyacinth" stones, greens, and colorless. The saturated cognac zircon pictured above is exactly what the material looks like out of the ground in the best Cambodian and Tanzanian deposits. Blue zircon is produced by heating brownish rough, usually from the Ratanakiri region of Cambodia, in a low-oxygen environment. The treatment is standard, stable under normal wear and always assumed for blue material. One caveat worth knowing: the color of some heat-treated zircons can shift back toward brownish with prolonged exposure to strong sunlight or ultraviolet, so a blue zircon is happier in a box than on a windowsill. Zircon is one of the species we discuss in our yellow gemstones guide too, where its natural colors get the attention they deserve.

Zircon durability

Zircon is hard enough for jewelry at 6 to 7.5 on the Mohs scale, but it is brittle, and the sharp facet junctions can abrade over years of ring wear into what the trade calls "paper wear". It also has a high specific gravity, so a one-carat zircon looks smaller than a one-carat sapphire; our carat weight versus size article explains why, and why you should buy zircon by millimeter rather than by carat. For a ring, choose a protective setting; for earrings and pendants, wear it without a second thought. Warm soapy water is the safe way to clean it. Explore our current loose zircons or learn more on the zircon page.

Turquoise: The Original December Birthstone

Turquoise is a hydrated copper aluminum phosphate, opaque, and colored by copper. It has been mined and worn since antiquity, from Persia (the Nishapur deposits in present-day Iran gave the world the "Persian blue" benchmark) to Egypt, China and the American Southwest, where the Sleeping Beauty mine in Arizona became famous for clean, even, sky-blue material with no matrix. The most valued color is an even, saturated robin's-egg or sky blue; greener stones, caused by iron, are generally less expensive, though many people love them. The dark veins running through many turquoise pieces are host rock, called matrix, and a fine "spiderweb" pattern can be prized in its own right.

What to ask before buying turquoise

Turquoise is soft, 5 to 6 on the Mohs scale, and porous. Because natural untreated turquoise can absorb skin oils, cosmetics and perfume and change color over time, most turquoise on the market has been stabilized, meaning impregnated with a polymer or resin to harden it and lock the color. Stabilization is accepted and legitimate when disclosed. What you want to avoid is undisclosed dye, "reconstituted" turquoise made from powdered fragments bonded with resin, and outright imitations such as dyed howlite or magnesite. The right questions are simple: is it natural, is it stabilized, is it dyed, and where is it from. A reputable seller will answer all four without hesitation. At Sapphire Row our own focus is faceted transparent gems, so tanzanite and zircon are the December stones you will find in our cases; if turquoise is what you are after, tell us through a sourcing request and we will point you in the right direction.

Which December Birthstone Should You Choose?

Ask what the piece is for. If it is a ring that will be worn every day, zircon in a protective setting is the most practical of the three, with turquoise the least. If the piece is a pendant, earrings or an occasional ring, tanzanite gives you the most color for the money and a story no other stone can tell. If you want a gem that will change how people think about "blue", a top zircon in the right cut will out-sparkle almost anything. And if the person you are buying for loves texture, history and a stone that looks as good in silver as in gold, turquoise is the answer.

On shape: tanzanite's trichroism shows best in shapes with a large open table and full pavilion, so ovals, cushions and trillions do it justice, and a well-cut trillion like the one at the top of this article throws blue and violet from the same face. Zircon rewards brilliant styles that maximize its dispersion. If you are building a piece from scratch around either stone, our bespoke service starts with the gem and designs the setting to protect it. For a wider look at the blue family, including sapphire and aquamarine, read our guide to blue gemstones.

December's three stones are a reminder that the word "blue" contains multitudes. Whichever you choose, buy from someone who can tell you exactly what the stone is, what has been done to it, and how to keep it beautiful, and you will have a birthstone worth passing on.

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