Tanzanite and Heat Treatment
Tanzanite is a blue-violet gemstone belonging to the zoisite family. Low-temperature heat treatment is a commonly used process to attempt to enhance the color of these gems. It is important to note that most tanzanites available on the market have undergone this treatment, but not all gems show improvement, remaining unchanged in some cases. This process is widely accepted in the gemstone industry and is considered normal and non-detrimental to the quality of the stone.
Once heated, some Tanzanite specimens exhibit intense blue-violet hues and are of the highest quality, while others retain lighter violet colors and are considered less valuable.
The presence or absence of low-temperature heat treatment is not a useful parameter for defining the quality of Tanzanites, unlike the color and transparency of the crystal the gem exhibits.
Here’s how the heat treatment of tanzanite works: Tanzanites are subjected to heating at low temperatures ranging from 350 to 550 degrees Celsius. This process is carried out in specialized ovens, and after heating, the stones are slowly cooled to prevent the formation of internal tensions. Note: Heating processes can also occur naturally in the later stages of crystal formation.
What happens, does the color change? After heat treatment, the color of tanzanite can improve. Many tanzanites that exhibit brown streaks due to the presence of oxides can retain only the blue/purple color after treatment. This is due to the removal of oxides and iron impurities present in the stone. If the gem does not contain such impurities, it will essentially maintain its original color.
Does clarity improve? Since they are heated at low temperatures, there are no significant changes in clarity. Traces of this treatment can be invisible, unlike high-temperature treatments applied to other gemstones such as corundums. In the latter, inclusions or small surface fractures are eliminated, but they can produce decryption halos or liquefaction of crystalline inclusions, which can be detected under a microscope.
Can the gem change over time? The elimination of iron oxides is permanent, and the gem will remain unchanged over time.
Detecting Heat Treatment in Tanzanites
Detecting this treatment can be very difficult, and specialized gemological tools and experience in gemstone analysis are necessary. However, very often, after careful analysis, no significant traces of the treatment are found. Following the guidelines of many laboratories, including GIA, even the LUXEDO laboratory does not specify heat treatment on tanzanite certificates unless physical evidence has been found.
Methods of Analysis:
- Gemological Microscope: A gemological microscope provides an enlarged image of the stone and allows for the examination of inclusions and other internal characteristics of tanzanite. Heat-treated tanzanites may display inclusions or features that appear “melted” or “glassy” due to the treatment.
- Spectroscope: A spectroscope can be used to analyze the absorption spectrum of light passing through tanzanite. Heat-treated tanzanites may exhibit specific absorption bands, often associated with the removal of iron impurities during treatment.
- Refraction and Reflection: The refractive index and the phenomenon of light reflection inside tanzanite can vary depending on the heat treatment. Instruments like the refractometer and the polariscope can be used to analyze these aspects.
- Ultraviolet Light (UV): Exposure of tanzanite to ultraviolet light can reveal certain features, such as fluorescence or phosphorescence, which may vary depending on the heat treatment.