Understanding Tanzanite’s Colour and Heating Process

by Gemological Identification Laboratory

The Fascinating Journey of Tanzanite

Tanzanite is a unique gemstone that captivates many with its stunning hues. It initially crystallizes with a brownish body colour. When exposed to moderate heat, this brown component diminishes. This process allows the stone's blue and violet hues to emerge more prominently.

A common misconception is that all tanzanite is mined solely in this brown state. In reality, many crystals are recovered displaying a blend of blue and brown coloration. This natural variation occurs because certain tanzanite crystals formed or resided closer to geothermal activity within the Earth. They may have also experienced metamorphic heating during their 500-plus million-year geological history. Such exposure results in enhanced colour saturation through natural thermal processes.

When tanzanite lacking this natural exposure is heated after mining, the process effectively replicates the same geological mechanism responsible for colour enhancement in naturally heated stones.

Colour Change in Tanzanite

Tanzanite exhibits a remarkable ability to change colour under different lighting conditions. This characteristic makes it a sought-after gemstone among collectors and traders. The colour change can be subtle or dramatic, depending on the light source.

In natural daylight, tanzanite often appears blue or violet. Under incandescent light, it may take on a more purplish hue. This versatility adds to its appeal, making it a favourite for various types of jewellery.

Heating Tanzanite

In the gemstone trade, it is generally understood that all tanzanite has undergone some form of gentle heating. This heat exposure may occur naturally while the crystal remains underground. Geological forces, such as volcanic activity, can cause this. Alternatively, similar low-temperature heating may be applied after mining to achieve the same effect.

It is important to recognize that heating does not automatically result in stronger or more vivid colour. Many tanzanite stones with lighter, pastel tones do not develop deeper saturation even after heat exposure. Currently, there is no scientific method capable of conclusively determining whether the heating occurred naturally or was carried out by human intervention.

Because all tanzanite is believed to have undergone heating at some stage—whether through natural geological processes or deliberate treatment—this factor does not influence market value. As a result, finished tanzanite gemstones are universally regarded as having been heated.

The gentle heating applied to tanzanite—typically around 600 °C for a relatively short duration—is fundamentally different from the high-temperature heat treatment commonly used for gemstones such as sapphire. Sapphires are often subjected to temperatures exceeding 1,700 °C, close to their melting point, and may remain under heat for days or even weeks. Such extreme conditions can significantly alter both colour and clarity.

Because the heating used for tanzanite is mild by comparison, it primarily serves to reveal colour rather than radically modify the gemstone. Major gemological laboratories, including GIA and GIL, describe tanzanite on certification reports as “heated” rather than “heat treated.”

Understanding the Value of Tanzanite

The value of tanzanite is determined by several factors. These include colour, clarity, cut, and carat weight. The most prized stones exhibit a deep blue or violet colour. Additionally, clarity plays a significant role. Stones with fewer inclusions are more valuable.

The cut of the gemstone also influences its appearance and value. A well-cut tanzanite will reflect light beautifully, enhancing its colour and brilliance. Carat weight is another essential factor. Larger stones are rarer and, therefore, more valuable.

For those looking to buy or sell tanzanite, understanding these factors is crucial. It helps in making informed decisions and ensures fair transactions.

Conclusion

Tanzanite is a captivating gemstone with a rich history and unique characteristics. Its colour-changing ability and the gentle heating process contribute to its allure. Understanding these aspects is essential for anyone involved in the gemstone market.

By recognizing the factors that influence tanzanite's value, individuals can confidently navigate their gemstone journeys. Whether you are a collector, trader, or simply an admirer, tanzanite offers a world of beauty and intrigue.

For more information on gemological identification and certification, visit GIL.