The final month of the Gregorian calendar is uniquely associated with three distinct gemstones: tanzanite, turquoise, and zircon. Each possesses a singular character, from the vibrant blue of a heat-treated zircon to the opaque, sky-blue history of turquoise and the mesmerizing color-change property of tanzanite. For individuals born in December, this triad offers a remarkable spectrum of choices, each with its own geological origin, historical significance, and gemological profile. The modern birthstone list for December, as established by the National Association of Jewelers in 1912 and updated over the years, reflects a balance between traditional and contemporary preferences, culminating in the inclusion of tanzanite, a gemstone that did not enter the commercial market until the 1960s. This article explores the science, history, and symbolism of these three December birthstones, providing a comprehensive guide for gemstone enthusiasts, collectors, and those seeking to understand the stones linked to their birth month.
History and Lore: From Ancient Talismans to Modern Marvels
The journey of the December birthstones through human history is a testament to their enduring appeal. Turquoise holds the distinction of being the most ancient of the three, with a documented history of use spanning millennia. In the Southwestern United States, Native American cultures were employing turquoise as a gemstone over 2,000 years ago, creating beads, pendants, intricate inlay work, and small sculptures. This deep-rooted connection ensures that turquoise remains a vital element of Southwestern cultural identity to this day. Its historical significance extends beyond the Americas; as an opaque mineral prized for its distinctive blue to blue-green color, it was often set into sterling silver jewelry, a pairing that highlights its natural beauty and has become a classic combination in jewelry design.
Zircon, the second December birthstone, has a history that is both long and somewhat complex. It is a gem with a high luster and a fire that can rival that of diamond, which led to its historical use as a diamond alternative. For centuries, zircon was valued for its visual properties, though its true identity was sometimes confused with other stones. The gem’s journey into the modern birthstone list was solidified in 1952 when it replaced lapis lazuli on most official lists. This change reflected evolving market trends and gemstone availability. At that time, topaz was a relatively costly gem, and the subsequent discovery of large deposits in Brazil greatly reduced its price, influencing birthstone selections. Zircon's own value and availability were also impacted by mining discoveries, particularly in Southeast Asia, which made its variety of colors more accessible to the public.
Tanzanite is the newest addition to the December birthstone family. It is a gemstone of the modern era, first discovered in 1967 in the Merelani Hills of Tanzania. Its rapid ascent to prominence is a fascinating story of geological serendipity and marketing. Most tanzanite sold in jewelry today exhibits a captivating blue to bluish purple to bluish violet color, a hue that is almost always the result of heat treatment applied to the naturally brownish material. The stone’s most distinctive characteristic is its trichroic property, meaning it displays different colors when viewed from different angles—a feature that has contributed significantly to its allure. As the newest member of the December trio, tanzanite represents a contemporary choice for those born in the final month of the year, offering a unique color and story that is distinctly 20th-century.
Geological Formation and Sources
The distinct origins of tanzanite, turquoise, and zircon underscore the diverse geological processes that create gem-quality minerals. Each stone forms under different conditions and is sourced from specific, often limited, locations around the globe.
Tanzanite is a variety of the mineral zoisite and is noted for its extreme rarity. It is mined in only one known location in the world: the Merelani Hills of northern Tanzania, near the base of Mount Kilimanjaro. This singular source makes tanzanite one of the rarest gemstones, a fact that contributes to its value and desirability. The gem is typically found in hydrothermal veins within the host rock, and its natural color is often a brownish hue that is transformed into its signature blue by a process of heat treatment, a common and accepted industry practice.
Turquoise is a secondary mineral that forms in the arid regions of the world where acidic, copper-rich groundwater percolates through rocks containing aluminum and phosphorus. The Southwestern United States is a historically significant source, with deposits in states like Arizona, New Mexico, and Nevada yielding gem-quality material. The color of turquoise varies from blue to blue-green to yellowish green, influenced by the specific mineral content of the host rock. The most prized color is a bright, even blue. The stone's formation process means it is often found in nodules or veins, which is why it is commonly cut into cabochons, sometimes in freeform shapes that maximize the use of the rough material.
Zircon is one of the oldest minerals on Earth, with crystals dating back billions of years. It forms in igneous and metamorphic rocks. While it is found in many locations worldwide, including Australia, Cambodia, and Sri Lanka, the gem-quality material used for jewelry often comes from specific deposits. The blue zircon that dominates the market is typically produced by heating brown zircon crystals—a treatment that is stable and permanent. The sources of the raw material for this treatment are varied, but the final product is a consistent and popular gemstone in the jewelry industry. The gem’s formation in high-temperature environments gives it exceptional durability, which will be discussed further in the gemological properties section.
Gemological Properties
Understanding the physical and chemical characteristics of these gemstones is essential for appreciation, care, and valuation. The following table summarizes the key gemological properties of the three December birthstones, based on established data.
| Gemstone | Mineral Species | Chemical Formula | Typical Color(s) | Mohs Hardness | Key Optical Properties | Common Treatments |
|---|---|---|---|---|---|---|
| Tanzanite | Zoisite | Ca₂Al₃(SiO₄)₃(OH) | Blue, bluish purple, bluish violet | 6.5 - 7 | Trichroic (shows different colors from different angles) | Heat treatment (to achieve blue color from brownish material) |
| Turquoise | Hydrated phosphate of copper and aluminum | CuAl₆(PO₄)₄(OH)₈·4H₂O | Blue, blue-green, yellowish green | 5 - 6 (low hardness) | Opaque; often mottled or veined | Stabilization (impregnation to improve durability and color) |
| Zircon | Zircon | ZrSiO₄ | Blue (most popular), yellow, red, brown, green, white | 6.5 - 7.5 | High luster, high refractive index, high dispersion (fire) | Heat treatment (for blue color from brown), irradiation |
Tanzanite exhibits a Mohs hardness of 6.5 to 7, making it relatively durable for jewelry, though it requires care to avoid scratches and impacts. Its most remarkable property is its trichroism. A well-cut tanzanite will display a primary blue color, with secondary hues of purple and burgundy visible from other angles. This dynamic visual effect is a hallmark of the stone and a key factor in its selection by jewelers and consumers.
Turquoise is significantly softer, with a Mohs hardness of only 5 to 6. This low hardness, combined with its porous nature, makes it susceptible to scratching and damage. It is also prone to breaking due to its lack of cleavage and tenacity. For these reasons, turquoise is almost always cut into cabochon shapes for use in jewelry. The stone's opacity means that light does not penetrate it, and its beauty is derived from its color and often intricate matrix patterns (the host rock included within the gem). To enhance its durability and color stability, much of the turquoise on the market is stabilized—a process where the stone is impregnated with a clear resin or wax. This is a standard and accepted treatment.
Zircon is one of the harder gemstones, with a Mohs hardness ranging from 6.5 to 7.5, making it suitable for all types of jewelry, including rings. It possesses a very high refractive index, which contributes to its brilliant luster, and a high dispersion, which gives it a "fire" similar to that of diamond. These properties made it a popular diamond simulant in the past. Natural zircon is typically yellow, red, or brown. The vibrant blue that is most popular in the market today is almost always the result of heat treatment, which is stable and does not fade over time. Zircon is also pleochroic, meaning it can show different colors from different crystal directions, a property that skilled cutters must consider to maximize the stone's color in the final gem.
Symbolism and Metaphysical Beliefs
Across cultures and centuries, gemstones have been imbued with symbolic meaning and metaphysical powers. The December birthstones each carry a rich tapestry of associations, from ancient healing beliefs to modern interpretations of personal qualities.
Turquoise is arguably the stone with the deepest symbolic roots in the Western Hemisphere. For the Native American tribes of the Southwest, it is not merely an ornament but a sacred stone, a piece of the sky and water brought to earth. It has been used as a talisman for protection, health, and good fortune. In broader historical contexts, turquoise has been associated with wisdom, tranquility, and protection from negative energy. Its color, reminiscent of water and sky, naturally lends itself to symbolism of peace, clarity, and spiritual grounding. The belief in its protective powers is one of the oldest and most persistent in the lore of gemstones.
Zircon, while less steeped in ancient myth than turquoise, has its own set of associations. Due to its diamond-like appearance, it has been linked to themes of purity, clarity, and prosperity. In some traditions, zircon is believed to promote honor, purity, and wisdom. The blue variety, which is the most common in jewelry, is sometimes associated with peace and tranquility, aligning with the calming blue hues that are popular in December birthstone selections. Its high luster and fire have also led to associations with enhancing one's self-esteem and confidence.
Tanzanite, as a newcomer, has quickly developed its own modern symbolism. It is often associated with transformation, a fitting metaphor for its own geological and commercial journey from a brownish rock to a celebrated blue gem. It is said to promote a higher consciousness, intuition, and spiritual perception. The stone’s unique color-changing property is sometimes interpreted symbolically, representing the ability to see multiple perspectives or to access different states of being. For those born in December, tanzanite is often viewed as a stone of new beginnings and visionary thinking, reflecting its status as a 20th-century discovery with a bright future.
Care and Cleaning
The practical care of gemstone jewelry is dictated by its physical properties, particularly hardness and stability. Each of the December birthstones requires specific handling to ensure its longevity and beauty.
Tanzanite, with a hardness of 6.5 to 7, is moderately durable but should be protected from hard blows and sharp impacts. It is best cleaned with warm, soapy water and a soft brush. Ultrasonic cleaners and steam cleaners are not recommended, as the vibrations and heat can potentially cause fractures, especially in stones with inclusions. Given its trichroism and the importance of a precise cut, it is also advisable to avoid exposing tanzanite to extreme temperature changes, which could stress the stone.
Turquoise demands the most careful handling of the three. Its low hardness (5-6) and porous nature make it highly susceptible to scratches, chemicals, and oils. It should never be exposed to cosmetics, lotions, perfumes, or household cleaners, as these substances can penetrate the stone and cause discoloration or damage the stabilizing treatment. Cleaning should be done with a soft, dry cloth. If necessary, a slightly damp cloth can be used, but the stone must be dried immediately and thoroughly. Prolonged exposure to water, heat, or sunlight should be avoided to prevent fading or cracking.
Zircon is the most durable of the December stones, with a hardness up to 7.5. It can be cleaned safely with warm, soapy water and a soft brush. While it is generally stable, some blue zircon may be sensitive to prolonged exposure to strong light or heat, though this is rare with modern treated stones. As with all fine jewelry, it is prudent to remove zircon jewelry during strenuous activities to prevent accidental impact or abrasion. Its high brilliance is best maintained with regular, gentle cleaning to remove oils and dust that can dull its surface.
Conclusion
The three birthstones of December—tanzanite, turquoise, and zircon—offer a compelling study in diversity. From the ancient, sky-blue talismans of the Southwest to the fiery, diamond-like zircon and the modern, color-changing marvel of tanzanite, each stone tells a unique story of geological formation, human history, and personal significance. Their inclusion on the modern birthstone list reflects a dynamic tradition that evolves with new discoveries and changing tastes. For those born in December, the choice between these three gems is not merely a matter of color preference but an opportunity to connect with a rich legacy of natural beauty and symbolic meaning. Whether drawn to the protective history of turquoise, the brilliant fire of zircon, or the transformative journey of tanzanite, individuals can find a gemstone that perfectly encapsulates the spirit of the year's final month.