Blue Zircon Question

[pandorian]

Hello,
I am new here. This is my first post
At a recent rock and mineral show, I spoke with someone who recently discovered naturally blue zircon (not heat treated).
Is there a gem name for the blue color of zircon?
What is the chromophore(s) for this color?
Thanks in advance for your help.

 

[soberguy]

My blue zircon was sold as unheated, but I was told that no blue zircon is unheated... so... lol... I dunno.

 

[VapidLapid]

I am under the impression that there is no way to diagnose a zircon for heating, at present. I believe the chromophore is iron, but im not sure. Michael E seems to know more about zircons and heating than anyone, hopefully he will chime in. Like soberguy,I have heard that all blue zircon are heated, but also that some that come out of the ground blue already are believed to have been heated in situ at some time after forming. Lots of confusion on a species if there are no tests to prove anything, so for the mean time it seems best to assume all zircons have been heated, at least once.

 

[Pandora II]

There are some natural blue zircons - certainly not a new discovery although rare - but 99.9% of blue zircons are heated. It is impossible to tell whether a zircon has been heated or not so I wouldn't pay extra for a natural.

Zircons IMO are a bit of a bugger - they contain uranium (one of the key give aways for IDing zircon is a uranium spectrum seen through a spectroscope (line in the red at 653nm). If you see that it's definitely a zircon, but sadly there are some zircons that don't have it - grrrr). Along with uranium they also contain thorium and hafnium all of which a radioactive compounds - over time these break down the crystal lattice through alpha particle decay eventually leaving an amorphous material (ie no crystal structure). This occurs over 10's of thousands of years.

Stones where this process has not being going on that long - or young stones - are called 'high' types. These are transparent, have high dispersion, and are golden, yellowish-green or greenish-brown in color.

Then there are 'low' type or 'metamict' zircons where the crystal lattice structure is broken down.

Between the two are the 'intermediate' type zircons.

High and intermediate type zircon can be found as colorless, blue, yellow, yellow-green to brownish-green and orangy-brown to orangy-red, pink and purple.

Low type zircon is generally cloudy brownish-green and yellowish-green but sometimes orange or brown.

I was given a stone to ID the other week which was a nasty greenish greyish brown (like a nasty green sapphire) and showed no double refraction through the loupe and was very dull in appearance - it's RI is too high to use a refractometer and while it was cloudy there were no inclusions that pointed to something else. I was going to hazard a guess that it was corundum with weird-looking 'silk' when I had a quick look with the spectroscope and the line at 653nm was as strong as any I've seen. It was indeed a zircon but it didn't look like any I'd seen before!

Heating zircons not only improves or changes the colour but, it can also turn metamict zircons back into high type by repairing the crystal lattice. Heating is carried out either in an oxidising or reducing atmosphere or both - one after another - till the desired colour is achieved.

Heating produces colourless, blue or golden and the orange to red colors. Heating orangy-brown to reddish-brown at around 900° C in an oxidizing atmosphere turns them yellow to red. At 1000-1400° C in a reducing environment it turns them into colorless or blue.
Any 'off' coloured stones are then reheated to around 900° C in an oxidising atmosphere turning them colourless, yellow, orange or red.

So, the sparkling zircon in your ring today could well be a lump of amorphous dark material in a few 10's of thousands of years from now - but just leave a note with your bling to remind your great/great/great etc etc etc grand-daughter to heat the thing up!

Exposure to light can also sometimes affect the colour of your zircon so don't leave it sitting in the sun!

As far as chromophores are concerned, Zircon is allochromatic and contains so many trace elements that at one time it was suggested naming it Polycrasolite (many things) and it is these that contribute towards the colouration of an individual stone.

Not sure is this has helped or been confusing... also if anyone spots an error here, please do point it out!

 

[VapidLapid]

Zowie Pandora, that's like wow!

 

[chrono]

Natural zircons occur in all shades of the colour spectrum. Initially colourless, it becomes coloured by mineral impurities, with the most common colour being yellowish brown. These brown rough are often heated with or without oxygen present to something colourless or blue and yellow. I think the heated blue zircons were unsuccessfully marketed as “starlite” a long time ago. Pandora is correct that some zircon crystals contain naturally radioactive thorium and uranium, which over time, breaks down the crystal structure into an amourphous glass-like state, with a lower RI and luster also known as metamict or low zircon. The link below has some interesting information on the impurities found in zircon.
http://www.thaiscience.info/journals/Article/La-icp-ms%20study%20of%20impurity%20ion%20concentrations%20in%20zircon.pdf

 

[Pandora II]

Are there are any zircons that don't contain uranium?

Also, although the RI is lower on the metamict zircons I'm pretty sure still above the RI of normal contact fluid. The one I was looking at the other week I did put on the refractometer but wasn't getting anything much visible other than a slight shadow edge for the contact fluid. I have been known to see lines all over the place on refractometers when a reading isn't straight-forward and especially with biaxial stones so I wasn't sure whether to place the stone as 'over the limit' despite the lack of good lustre or just blame my eyes not spotting some very faint lines! Anyone know an approx RI for metamict zircon is?

 

[VapidLapid]

High: 1.94-1.987

metamict: 1.777-1.79

 

[chrono]

Pandora,
I’m not sure how accurate this source is, but it lists the RI of the 3 states of zircons.
http://www.gemstonebuzz.com/zircon

 

[pandorian]

Wow,
You folks are awesome! Thanks much for all the valuable info, especially the "uranium" 653nm line.

As far as naturally occurring Blue zircon, I found the following photos from Italy in a web search:
http://www.mindat.org/gallery.php?min=4421&loc=2170

I am certain that the stuff I saw is unheated since the finder had pictures of the stuff in-situ. He did not think it was zircon (due to the blue color) until the chemical analysis. It does not come from any of the classical zircon localities.

Thanks again,
-Pandorian

 

[soberguy]

Here is mine, in a temp ring with 3mm rb diamonds... As you can see it's quite large, and has a fabulous blue color, not muddy, or too greenish.

 

[LD]

Pandorian - if the ones you've seen are a bright blue (like the photo of mine below), I would suggest that heating has occurred. I believe (somebody correct me if I'm wrong) that unheated blue zircons are paler? However, as Pandora quite rightly states, as there is no exact way of determining unheated/heated in Zircons, you most definitely must not pay a premium and it's always safer to err on the side of caution and assume heating.

 

[minousbijoux]

Okay, its official: Pandora, you are my new hero! Between this and your account of your moonstone trip to Sri Lanka, I am happy as a clam. Thanks for your amazing response.

 

[Edward Bristol]

I can’t match Pandora’s analysis but report from the front lines of the quest for untreated gemstones:

We have abandoned (and refunded) three hunts for unheated blue zircon over the years. Only recently have we found one green-blue zircon and sold already. I think the image was posted here somewhere earlier.