Inspired by the discussion on the live, I thought I'd put together a quick post on Tanzanite & heat treatment. It's a gem that's very close to my heart, as my engagement ring is actually a Tanzanite! As a side note - it's not especially practical for everyday jewellery due to it's hardness, but I absolutely adore it!
So, what is Tanzanite?
Tanzanite is a variety of Zoisite found in just one place on Earth: the foothills of Mount Kilimanjaro in Tanzania. Discovered relatively recently, in the 1960s, this gemstone has quickly become a favorite for both faceted jewellery and specimen collectors.
What Colour is Tanzanite Naturally?
In its natural, freshly mined form, Tanzanite often appears with a brownish, dull tone, not the bright, vivid violet blue that we're so used to seeing.
Why Heat Treat Tanzanite?
The purpose of heat treatment is simply to bring out the stone's beautiful blue and violet colors. By heating Tanzanite to around 500-600°C, gemologists can enhance the stone’s natural color, removing the brown-yellow tones and thereby making it more vibrant and desirable.
How Common Is Heat Treatment in Tanzanite?
Almost all Tanzanite on the market today has been heat-treated, and unless stated otherwise from a trusted seller, I would assume that it has been heated. This process is so widely accepted that untreated Tanzanite is incredibly rare and often lacks the vivid colors that make Tanzanite so special, often containing yellow shades. Unheated Tanzanite that retains the more muddy colouring is so much less desirable for jewellery that it is very rare that any pieces make it out of the mine, so trying to collect the yellower shades is actually really difficult!
The photo below shows why it is so incredibly difficult to differentiate between artificially heat treated, and natural high quality pieces. Both are photos from Mindat, credit to Rob Lavinsky at iRocks.
Why Is It an Industry Standard?
So why is heat treatment for Tanzanite such a common practice, and why is it so accepted, in comparison to other crystals like heat treated Amethyst that is sold as Citrine?
This I think is the more important takeaway from this post - it it heated to improve the colours of the stone, without then being passed off as something else. Yellow-brown Tanzanite is still Tanzanite, and after heating it's still called Tanzanite. It's not trying to be passed off as a completely different crystal.
For me, that is the dividing line - if Amethyst is heated, loses it's purple colour and becomes either yellow or green to be sold as Citrine or Prasiolite, I personally find that deceptive. If Clear Quartz is irradiated, and then sold as Citrine, the same. Gems like London Blue and Swiss Blue Topaz are always irradiated to achieve that colour, so where do you feel they fall? The heating of gems to remove colour impurities in gems is very common in the jewellery world, and is expected to a certain degree, but what are your thoughts when it comes to crystal and minerals as collection pieces?