Tanzanite (Blue Zoisite)
Hardness: 6.5
Ca2Al3(SiO4)3(OH)
硬度: 6.5
化学成分: Ca2Al3(SiO4)3(OH)
Tanzanite, a relatively new gem but has already gained quite a popularity thanks to Tiffany naming it after the place it was found. It was first found in 1967, a Masai tribesman stumbled upon a cluster of highly transparent, intense violet-to-blue crystals weathering out of the earth in Merelani, an area of northern Tanzania. He alerted a local part-time gold prospector named Manuel d'Souza, who thought it might be a sapphire, but later was identified by the British Museum and Harvard University as a blue zoisite.
Tanzanite has a unique property as it exhibits strong trichroism (display 3 different colours) blue, purplish-blue and reddish-brown when viewed in different axis. This can be used to differentiate from sapphire.
The mineral Zoisite was discovered in the Austrian Alps in 1805. It was named after the mineralogist Sigmund Zois, who first identified it. It comes in a wide range of colours that includes colourless, grey, yellow, brown, pink, green, blue, and violet. The name "tanzanite" is used for a colour variety of zoisite that ranges from blue to purplish-blue. The blue colour of tanzanite is caused by small amounts of Vanadium within the zoisite mineral structure. Most tanzanites obtain it’s blue or purplish-blue through heat treatment at temperatures between 370-600°C, (consider relatively low heat compared to sapphire) the oxidation state of the Vanadium is changed, removing the brown and improving the blue. Only a fraction of tanzanite obtains its ‘natural’ blue without treatment, through the heat of metamorphism or wildfire. This naturally blue, untreated tanzanite is held in very high regard by some gemstone collectors.
In fact, the rarest of all zoisite colours are pink with a crystalline form, that is less than 0.0001% of the total production. The colour pink is due to the trace element of Manganese, where most of the crystal is pretty included and the colour appears only on one axis, which is also the very reason why many dealers have never encountered it, hence any shade of pink is highly sought after by collectors. Not to be confused with Thulite which is an opaque massive form of pink zoisite that is relatively common. Some of the pink zoisites have a strong tint of orange or best salmon pink. That could due to a thin crystal that is being cut at an angle to get a certain depth, nevertheless, it is still considered desirable second to pure pink crystalline material.
If unheated tanzanite is rare, an unheated pink zoisite would be extraordinary.