By Lauriane Lognay
Considered rarer than diamond, tanzanite has been known for its rich blue to purplish colour range since its discovery in 1967. It remains one of the most sought-after gemstones in the industry, and can only be found in Tanzania near Mount Kilimanjaro. As tanzanite celebrates 50 years, let’s dive into the story behind it and how to take care of it for years to come.
About the gem
Although tanzanite is a relatively new gemstone—at least in comparison to lapis lazuli, for example, which was around in ancient Egypt—it is the main gem representing those born in December. The blue/purple gemstone is a variety of the mineral zoisite, which can be found in a multitude of colours, such as yellow, pink, green, and bicolours.
Having a hardness of 6.5 on the Mohs scale, blue zoisite is a relatively soft stone. It is also one of the rarest gems on the planet. It can often be confused with sapphire when the blue is saturated enough—however, most rough gems coming out of the mine are brown. Brown zoisite is mined and then heated to produce the blue and purple hues we see. Natural blue tanzanite is possible, but very scarce.
There is a simple test to determine if a tanzanite was heated or not, which is viable if you don’t have instruments with you (and if the specimen you have in your hands decides to co-operate with you). Natural tanzanite has what is called ‘pleochroism’ in gemmological jargon. Simply put, this is the ability of the stone/crystal to show different colours when viewed from different directions or angles. What is rare about Tanzanite is that it is trichroic (i.e. three-colour).
Most gemstones would not have any pleochroism, or would be dichroic and show only two different colours, but natural unheated tanzanites show three different colours at all times: blue in one direction, purple in another, and a third colour (generally magenta, green, brown, yellow, or a mixture of these). When the tanzanite is heated, the third colour disappears, and only the blue and purple are visible. Tanzanite is one of the few gemstones that can be trichroic or dichroic, making it easier to identify than some other gemstones.