Tanzanite granite. What is it?
Tanzanite is a trade name that was first used by Tiffany and Company for gem-quality specimens of the mineral zoisite with a blue color. Tiffany could have sold the material under the mineralogical name of "blue zoisite," but they thought the name "tanzanite" would stimulate customer interest and be easier to market.
The name "tanzanite" was given because the world's only known tanzanite deposit of commercial importance is in northern Tanzania. The name reflects the gem’s limited geographic origin. The mines are all located in an area of about eight square miles in the Merelani Hills, near the base of Mount Kilimanjaro and the city of Arusha.
Although nearly all of the world’s most popular gemstones have been known and used for hundreds of years, tanzanite was not discovered in commercial quantities until the 1960s. In the short time since then, it has become the second most popular blue gem after sapphire. It is one of a very small number of gems of any color that have been discovered and brought to strong consumer popularity within the past century. This rapid rise to popularity was accomplished mainly by Tiffany’s promotion and tanzanite’s beautiful blue color.
"Tanzanite granite is the most beautiful blue stone to be discovered in 2000 years." - Tiffany & Co.
natural blue tanzanite
Natural blue tanzanite granite: The pair of crystals shown on the left is the same pair of crystals shown on the right. They have different apparent colors because tanzanite is pleochroic - it appears to be different colors when viewed from different directions.
If we cut a faceted stone from the crystal pair on the right and oriented the cutting so that, in this view, we were looking down onto the top of the table, the stone would have a blue face-up color. If we did the same with the crystal pair on the left, the stone would have a purple face-up color. If the table of the stone were tilted slightly in any direction, an intermediate face-up color would be produced.
This pair of crystals has not been heated by humans. Their colors are natural. It is an example of tanzanite having spectacular natural color. Specimen and photo by Arkenstone / www.iRocks.com.
Tanzanite’s Interesting Color
The mineral zoisite naturally occurs in a wide range of colors that include colorless, gray, yellow, brown, pink, green, blue, and violet. The name "tanzanite" is used for a color variety of zoisite that ranges from blue to bluish purple to bluish violet. This type of color-variety name is not unusual. The name "ruby" is used for red to slightly purplish red specimens of the mineral corundum; the name "amethyst" is used for purple specimens of the mineral quartz; and, the name "emerald" is used for green specimens of the mineral beryl. Each of these minerals occurs in a wide range of other colors.
The discovery of transparent crystals of blue zoisite in the 1960s stimulated interest in the gem. Soon after that discovery, laboratory experiments determined that heating could improve the color of some naturally blue stones. They also determined that heating could convert some naturally brown or green zoisite into beautiful blue zoisite. With those discoveries, there was enough blue zoisite to support a marketing effort that would introduce the gem to millions of people.
The blue color of tanzanite is caused by small amounts of vanadium within the zoisite mineral structure. When vanadium-bearing zoisite is heated to a temperature of 600 degrees Celsius for about 30 minutes, the oxidation state of the vanadium is changed and that change causes or improves the blue color.
The heat treatment of tanzanite is very mild when compared to what is done for gems such as rubies and sapphires. Those gems are heated to temperatures between 1000 and 1800 degrees Celsius and held at those temperatures for days or weeks.
Today, nearly all of the gems being sold as "tanzanite" have a blue color that has been enhanced by heating. This treatment is always to be disclosed by the seller. A small amount of tanzanite in the marketplace has a blue color that was produced naturally through the heat of metamorphism without any treatment by people. This naturally blue, untreated tanzanite is held in very high regard by some gemstone and jewelry buyers who seek it out when making a purchase.