Tanzanite is one of the most intriguing and rare gemstones in the world. Tiffany and Company first used the trade name Tanzanite, instead of the mineralogical name "blue zoisite." Tanzanite is a variety of zoisite, and this gem is often described as a geological phenomenon. The gemstone varies from different shades of blue to purple.
It first made an appearance in the market when it was discovered in 1967. Before this, the public didn’t know anything about this gem. Now, tanzanite is extremely popular and valuable.
A Matter of Chemistry
The chemical composition of tanzanite is Ca2Al3Si3O12(OH) which is the same formula as zoisite since it’s a species of it. Its class is Silicates. The formula includes calcium, aluminum, hydroxyl and silicate.
It rates as a 6 – 6.5, sometimes up to 7 on the Mohs Hardness Scale.
How are Tanzanites Formed?
Even though tanzanite was only discovered in the late 1960s, it was first formed over 550 million years ago. When Mount Kilimanjaro was formed, the intense heat, pressure and tectonic plate activity created the Mozambique Orogenic Belt. This is known to have more minerals on earth than almost anywhere else. This is where tanzanite is believed to have been formed as well.
The process that created tanzanite is called regional metamorphism. It’s when the igneous rocks of the rolling tectonic plates morph into each other from the extreme pressure. The rocks are all nearly melting from the intense heat, but since they don’t reach the melting point, they form crystal structures instead. This is how many gemstones are formed. The tectonic plates each have different minerals, so that’s why unique combinations when the plates collide can occur.
Tanzanite is one of the unique combinations - it’s why it can only be found in a small area near the foothills of Mount Kilimanjaro. The mix from the tectonic plates that created tanzanite has never occurred or at least been found anywhere else on earth. And, likely, it will never happen again. Geologists have stated that there’s only a one in one million chance of another batch of tanzanite ever being created again.
This process of formation has a direct result on the value of this gemstone. There’s not an unlimited amount of it, and it does have the potential to be completely mined within a couple of decades or even less.
Occurrence of Tanzanite
There is only one country in the world where tanzanite is mined, and that is Tanzania. Specifically, the Merelani Hills of northern Tanzania, near the base of Mount Kilimanjaro. This is what makes it such a valuable gemstone. In 2002, it officially became one of the birthstones for December.
Tanzanite Valuation and Treatments
Tanzanite is valued based on the 4 C’s - Cut, Color, Clarity and Carat.
The cut is something that shouldn’t be overlooked when it comes to Tanzanite. With Tanzanite, cut isn’t only about the shape. It largely contributes to its grade, value and beauty. It’s important to choose a gem that wasn’t cut for weight retention. When this is done, it can cause a variety of problems ranging from loss of brilliance to windows along with bad symmetry.
The best cuts of tanzanite will have symmetrical faces without windows (areas inside the stone that look transparent or nearly white). The light of a good cut of tanzanite should bounce off the facets and shine, rather than leak through one part of the gemstone.
With tanzanite, the darker and more vivid the shade of blue or purple, the more rare and valuable it is. Larger stones usually have deeper colors, while smaller stones typically have pastel shades. All shades of tanzanite are a mix of both blue and violet shades. For example, tanzanite can be violet with a tint of blue, or blue with a tint of violet.
The standard clarity for tanzanite is flawless. The stones should be completely clean to the eye. If you can see inclusions or cracks within tanzanite, the value of that gemstone should be lower.
With tanzanite, the carat weight doesn’t always refer to the same size of gemstone. You can have two gemstones that have the same carat weight but don’t look anything alike. This is because the carat weight can change with the different variations in the gemstone’s proportion. Thus, measuring the diameter of tanzanite’s length and width is recommended for getting an accurate size.