Tanzanite Untreated vs. Heated: A Comprehensive Guide to Quality and Value

Introduction: The Phenomenon of Tanzanite

Tanzanite, the blue-violet variety of the mineral zoisite, was introduced to the world in 1967 in the Merelani Hills of northern Tanzania. Its striking colors, ranging from deep sapphire blue to vibrant violet, quickly captured the attention of gem enthusiasts and collectors. However, the story of tanzanite is not just about its beauty; it also involves a fundamental treatment that shapes nearly every stone on the market. This article explores the differences between untreated and heat-treated tanzanite, how heat affects color and clarity, and what consumers should know when purchasing this one-of-a-kind gemstone.

The Origin and Geology of Tanzanite

Tanzanite is found exclusively in a small mining area near Mount Kilimanjaro in Tanzania. The gemstone forms in metamorphic rocks, specifically in the presence of vanadium, which is responsible for its coloring. The geological conditions of the Merelani Hills are unique, and no other source of gem-quality tanzanite has been discovered elsewhere. This exclusivity contributes to its allure and value, but it also means that all tanzanite comes from a single, finite region.

What Makes Tanzanite Special?

Tanzanite is a variety of the mineral zoisite, with the chemical formula Ca2Al3(SiO4)3(OH). It exhibits strong pleochroism, meaning it displays different colors when viewed from different crystal directions. In its natural state, tanzanite can appear blue, violet, or burgundy-brown, depending on the angle. The most prized color is a deep, saturated blue with a subtle violet undertone, a color that is rarely found without treatment.

The Role of Heat Treatment in Tanzanite

Heat treatment is an industry-standard process used to enhance the color of tanzanite. When tanzanite is mined, most rough stones are brownish or yellowish due to the presence of vanadium in a trivalent state. Heating the stone to temperatures between 370°C and 390°C (approximately 700°F to 735°F) reduces vanadium to a tetravalent state, which alters the crystal lattice and produces the intense blue and violet hues that are so desired. This process is stable and does not require additional treatments like oiling or resin filling.

Is Heat Treatment a Natural Process?

Yes, heat treatment mimics and accelerates natural geological processes. In nature, some tanzanite may be heated by volcanic activity or metamorphism over millions of years, but such naturally heated stones are extremely rare. The vast majority of tanzanite on the market, estimated at over 95%, has been heat-treated in laboratories. It is important to note that heat treatment is a permanent and standard practice, and it is fully disclosed in the gem trade. Untreated tanzanite is quite rare and is often sold at a premium due to its scarcity.

Comparing Untreated and Treated Tanzanite

When comparing untreated and heat-treated tanzanite, several factors come into play:

• Color: Untreated tanzanite typically displays a brown or greenish-brown color that is not highly prized. Heat-treated stones exhibit the classic blue-violet spectrum.
• Clarity: Heat treatment can improve clarity by reducing the visibility of silk or included minerals, although it may also cause certain inclusions to become more evident.
• Stability: Both untreated and heat-treated tanzanite are stable under normal wearing conditions, but heat-treated stones may be slightly more sensitive to sudden temperature changes.
• Value: Because untreated stones with desirable blue color are extremely rare, they command higher prices. However, an untreated stone with brownish color is less valuable than an attractive heat-treated stone.

The Science Behind the Color Change

Color in tanzanite arises from vanadium ions occupying zirconium sites in the crystal structure. In unheated stones, vanadium exists as V3+, which causes absorption of light in the yellow-green region, resulting in a brown or grey appearance. Heating provides energy that enables V3+ to lose an electron and become V4+. This new electronic state alters the absorption spectrum, now absorbing light in the red and green regions, which transmits blue and violet wavelengths. The heating process is carefully controlled to achieve the desired depth of color without causing fractures.

How to Identify Untreated Tanzanite

Identifying untreated tanzanite can be challenging for the average buyer, but gemologists use a few methods:

• Pleochroism: Untreated tanzanite often shows a brown or yellow hue in one direction, while heated stones show blue and violet shades.
• Inclusion Patterns: Unheated stones may contain inclusion features that are altered by heat, such as "fingerprint" inclusions or healed fractures.
• Color Zoning: Natural unheated tanzanite may have uneven color distribution, while heat-treated stones tend to have more uniform color.
• Spectroscopic Analysis: UV-Vis spectroscopy can detect the presence of V4+ and other transition metals, providing a definitive answer.

If you are considering purchasing a tanzanite, especially one labeled as untreated, always request a gemological certificate from a reputable laboratory such as GIA or AGS.

Buying Considerations and Market Value

When shopping for tanzanite, the first question is often: should you seek untreated or heat-treated stones? The answer depends on your budget and preferences.

Unheated Tanzanite

Untreated tanzanite with natural blue color is exceptionally rare. Most unheated stones are brownish or pale and are not considered attractive by commercial standards. However, a few naturally blue stones have been found, and these are highly collectible. Prices for such stones can be significantly higher than their heat-treated counterparts, sometimes double or triple the per-carat price. If you are an investor or a collector, an untreated stone with fine color and clarity might be a worthwhile acquisition.

Heat-Treated Tanzanite

The vast majority of tanzanite in jewelry is heat-treated. This treatment is accepted and respected in the trade, much like the heat treatment of rubies and sapphires. Heat-treated tanzanite offers the vibrant blue-violet colors at accessible prices, making it a popular choice for engagement rings and fine jewelry.

When purchasing heat-treated tanzanite, pay attention to the depth of color. Look for stones with a saturated blue-violet hue, free of zones of brown or grey. Clarity is also important; stones with visible inclusions or fractures are less durable. Always ask the seller for a disclosure of any treatments, and be wary of misrepresentation.

Caring for Tanzanite

Tanzanite ranks 6.5 to 7 on the Mohs scale of hardness, making it suitable for earrings, pendants, and occasional wear rings, but it is more susceptible to scratches than sapphire or diamond. To care for tanzanite:

• Avoid ultrasonic cleaners and steam cleaning, as these can damage the stone.
• Clean with warm, soapy water and a soft brush.
• Store away from harder stones to prevent scratching.
• Protect from sudden temperature changes, as tanzanite can be thermally shocked.

With proper care, tanzanite can last a lifetime, but it should be reserved for jewelry that is not subjected to rough wear.

The Future of Tanzanite

As the only known source of tanzanite, the Merelani Hills are not inexhaustible. Estimates suggest that the mines may be depleted within a few decades, which could drive prices up in the future. This potential scarcity adds an investment appeal to tanzanite, especially for high-quality specimens. However, for the average consumer, the beauty and affordability of heat-treated tanzanite remain the primary draw.

Conclusion

Tanzanite, with its captivating colors and single-source origin, is a gemstone that deserves attention. Understanding the difference between untreated and heat-treated tanzanite is crucial for anyone looking to buy or appreciate this unique stone. While the majority of tanzanite is heat-treated to reveal its finest colors, untreated stones hold a special place for collectors due to their rarity. Regardless of treatment, tanzanite offers a dazzling blend of blue and violet that is unmatched in the gem world. As with any gem purchase, education is your best tool. Know what you are buying, ask for proper disclosure, and choose a stone that speaks to your heart and budget.