Abstract： Tanzanite is also known as zoisite. It is a type of silicate mineral and it was first used as decorative material. Since the discovery of blue-violet transparent crystal of tanzanite in 1967 Tanzania, its status in gemstone industry has been enhanced. To celebrate the establishment of United Republic of Tanzania at the time, the gemstone was named tanzanite. In some countries, tanzanite is called “Danquan gemstone”, the transliteration of tanzanite, which means “red fountain”in Chinese.

Keywords: Physical and chemical properties of tanzanite, coating treatment and identification

Chemical components: Ca₂Al₃(SiO4)₃(OH), but may also include V, Cr, Mn and other elements. Crystal system: orthorhombic system.

Crystal habit: columnar or plate columnar crystals. Cleavage: {100} perfect cleavage in one direction. Moh’s hardness: 6-7. Density: 3.35(+0.10，-0.25)g/cm³.

Tanzanite is usually in greenish blue with a little bit of brown. Gray, brown, yellow, green and pink ones can also be found. After heat treatment, the colors between brownish green and grayish yellow are removed from tanzanite, and the gem shows blue or bluish violet with vitreous luster. It has strong trichroism. The pleochroic colors of green tanzanite are blue, amaranth, and greenish yellow. The pleochroic colors of brown tanzanite are green, violet, and light blue. The pleochroic colors of yellowish green tanzanite are dark blue, yellowish green and violet. Refractive index: 1.697-1.700. Biaxial crystal.

As the beauty of tanzanites are deeply appreciated and widely cherished, the market now is not without treated tanzanites to meet the consumers’ needs.

Most tanzanites in the market have experienced heat treatment. This is a well-known fact and widely accepted. However, now there are coated tanzanites in the market. Our laboratory has received one setting tanzanite with its pavilion coated. It is difficult to identify from the first sight, because the coating cannot be recognized with naked eyes. Its pleochroism, refractive index, gravity and other basic properties are the same to unprocessed tanzanites in the tests.

But through loupe magnification, white marks can be found on its pavilion, which looks like gray layers but cannot be wiped out. Coating fall off can be observed on facets and lines, and the color is not homogeneous.

Coating is on the surface and coating fall off is obvious

It is highly possible that coated tanzanite may shed color after long-time ultrasonic cleaning. The original material of coated tanzanite is usually light-colored, poor-quality and valueless, and can be identified through tests in the lab. If a coated tanzanite is re-polished, its color will significantly fade.

F.T.I.R test

F.T.I.R testing cannot be used to identify whether a tanzanite is coated or natural, because the two show no difference in the F.T.I.R spectrogram.

Conclusion

Currently, enhance treatment is common in the gemstone industry. More than half of the gemstones in the global market are treated.

Enhance treatment can help to better utilize the existing natural gemstone resources, but it may also degrade the stability of some gemstones and affect their value. Consumers should be cautious when purchasing the gemstones, and are strongly recommended to get an authoritative identification report.