Tanzanite fading in Sunlight?

Jewelry Discussion
41 
With all due respect, please do a bit more research. Tanzanite, as
it comes out of the ground at every mine I've ever heard about, at
least according to the miners I've heard, including the late
Cambell Bridges when he talked about it at a Tucson show many years
ago, is not purple.

OK. Time to eat some crow here. I apologize for the above assertion,
and the same in a few other posts. Doing a bit more digging, after
having so ademently stated that all Tanzanite is heat treated, I’m
reminded that while the vast majority of it is indeed heat treated,
there are indeed a few that come out of the ground not needing
treatment. Mostly, these are, or were, stones found near the surface
of the deposits, especially early in the mining history of the
stone, or so I’m informed. Presumably, my statement that the geology
of formation of the stone doesn’t heat treat it already, isn’t
correct, since something made sure the brownish/orange trichroic
color didn’t appear, but now I’m on thin ice. Don’t really know any
more…

One thing my current sources do still seem to confirm is that there
seems no way to prove that a stone has not been heat treated, or at
least, no such test seems published that I can find, and from what I
can see, there seems no premium in market price for stones asserted
or known to be unheated. That may not apply to all cases and every
stone, but there doesn’t seem to be any prevalent preference for
unheated Tanzanite, perhaps because there is so little, it can’t be
proved, and seems to make no difference in the end stone. But again,
now I’m relying on conjecture.

I can say that my prior assertion was based on good memory of a talk
by the late Cambell Bridges in the '80s (some time ago, I agree…),
as well as discussions with at least two other dealers who I know to
be directly associated with mining of the stone. But perhaps what I
was told was simplified or the rare exception not mentioned.

Sorry 'bout that.

So then. Is crow best served grilled, fried, or roasted? What’s a
good sauce?

Peter Rowe

42

Thank you for answering my questions.

  1. What is the source of your ?
  2. Can you describe the process causing Tanzanite to fade ?
  3. Why are you singling out calcium as an admixture responsible for
    gemstones instability in the sunlight ?
1) AGTA ( as I said in the post), IGTA etc. I also have a friend
in Tanzania that mines tanzanit

On question 1: can you more specific ?

On question 2 and 3:

2) sunlight 

3) calcium containing stones seem to fade faster than stones
without it

Until you provide specific reference, I would have to say that you
are misinformed. Calcium is major component of grossular garnets
(tsavorite is one of them) and they are stable in the sunlight.

Fluorite acquires it’s violet color after been exposed to sunlight
due to presence of calcium. In fluorite case, calcium plays positive
role. It is wrong to assume that presence of calcium will
automatically doom gem to fading.

In you answer to question 2 you lack specifics. I was hoping for
detailed description of the fading process.

I wonder if you aware that there are coated tanzanites on the
market. I suspect that rumors of tanzanite fading connected to these
coated stones.

Leonid Surpin

43 
I have a 7+ caret, square brilliant stone which I faceted from
unheated material.

Can you post a picture of the brilliant stone, and the stones you
heated. Maybe it would show the differences to the interested party.

Heidi

44 
If not, assume the stone, like most rough, was already heat treated
properly when you bought it. So far as I know, most of the heat
treatment of tanzanite is done by the miners or soon after. among
other things, the final color cannot be accurately judged till
after treatment, so they don't know exactly what to charge till
they know what they have. At least, that's my understanding of the
situation.

That’s also my understanding. I’ve seen many documentary bits of
footage from more than one Tanzanite mine, and in each case, they
said that after the Tanzanite rough was separated from the unwanted
rocks (by using a certain liquid which made the rubbish float and the
Tanzanite sink (I think it was that way round)), it was indeed heat
treated as the next stage of the process. None of these pieces of
film involved people picking bits of blue rough from the conveyor
belt as came out of the separating process and before it went for
heat treating. After heat treating, it was then sorted and graded,
etc.

Helen
UK

45 
Until you provide specific reference, I would have to say that you
are misinformed. [snip] I wonder if you aware that there are
coated tanzanites on the market. I suspect that rumors of tanzanite
fading connected to these coated stones.

I agree with Leonard here.

Much of this discussion seems to center on second-hand information
and factual mis

I’ve learned to never say “Never”, but in 35 years of cutting stones
I’ve never heard or seen a single instance of Tanzanite fading due to
sunlight, and I’ve bought, sold and cut a fair amount of it.

I just spoke with one of the original importers of Tanzanite, the
first to be selling it in Tucson (he made millions), and he has
handled many hundreds of thousands of carats of Tanzanite with no
mention or report of any fading.

I have a call in to Bear Williams, a friend and long-time member of
the AGTA board, gemstone vendor and owner of a rather sophisticated
gemstone laboratory, so we’ll see what he says.

Please keep in mind that there are a slew of look-alike Tanzanite
simulants out there, from lowly glasses to synthetic Forsterite (easy
mis-identification) to Tanzanite colored YAG (complete with blue to
purple color shift), doublets and coated stones of various sorts.

I’ve heated lots of brown-yellow zoisite to turn it blue, and it
doesn’t take 900 C to do it, you can do it in your home oven as I
have many times…

Reports of Tanzanite being purplish-blue when it comes out of the
ground are frequent, but I know of no person who has actually SEEN
it come out of the ground that color. It’s possible, but in any case
it does not affect value. And there is quite a range to the colors of
zoisite, including pink and green, but, by definition, Tanzanite is
the purple-blue variety.

Folks with little to no experience should be cautious in their
pronouncements here, it leads to much confusion among the Newbies.

Wayne Emery, believes it when he sees it
The Gemcutter
www.thelittlecameras.com

46 
OK. Time to eat some crow here. I apologize for the above
assertion, and the same in a few other posts. Doing a bit more
digging, after having so ademently stated that all Tanzanite is
heat treated, I'm reminded that while the vast majority of it is
indeed heat treated, there are indeed a few that come out of the
ground not needing treatment.

No need for an apology. Untreated tanzanite is so rare, possibility
of encountering one in a jewelry store is mostly theoretical rather
than practical.

Zoisite (tanzanite) suitable for heat treatment could be green color
due to presence of iron and chromium; pink (rosaline) due to presence
of manganese, and blue due to presence of vanadium. Pure colors are
exceedingly rare because more than one admixtures are present.

After treatment pink becomes light-blue, yellow-green becomes blue,
and greenish-blue-brown becomes highly prized violet-blue. It should
be noted that temperatures are different for each hue.

It is possible to have untreated blue tanzanite, but it is extremely
rare. Here is how one can tell the difference. Tanzanite is
trichroic: blue/ purplish-red/greenish-yellow. After heat treatment,
the greenish-yellow is either completely gone, or severely
diminished. Color of untreated tanzanite is different from treated,
but the difference is not diagnostic. It is only indicative.

Leonid Surpin

47

does any one else but me suppose the occurance of a forest fire
would causeenough heat to turn zoisite from brown to blue to a
certain depth in a surface deposit ?

for as long as the stuff has been lying in the ground one must
consider possibilities !

goo

48

From the AGTA Source Directory’s PDF on the disc supplied to
members:

  • Gemstone:Tanzanite

  • Treatment :E or H (the “E” indicates that the stone is routinely
    enhanced)

  • Heated to produce violet-blue color (implies that voilet blue is
    the only colour that requires heat treatment which corroborates my
    friend in Mt. Tsavo that mines it’s description of how his material
    comes out of the ground)

  • Stability: Usually Excellent

  • Special Advice: Avoid sudden temperature changes

49 
does any one else but me suppose the occurance of a forest fire
would causeenough heat to turn zoisite from brown to blue to a
certain depth in a surface deposit ?

good theory…probably quite a good explanation of some minerals
changing colour then being mined ( from relatively shallow or surface
collected areas) that have been slashed and burned… makes great
sense…

50

Recent posts regarding tanzanite stated that the occurrence of
naturally blue, unheated tanzanite was extremely rare. According to
an article in the March 2009 issue of Colored stone magazine however,
natural blue tanzanite is being found in “significant” quantities.

Jerry in Kodiak

51 
does any one else but me suppose the occurance of a forest fire
would causeenough heat to turn zoisite from brown to blue to a
certain depth in a surface deposit ?

You never know Goo. Someone mentioned the other day that you could
change the colour of tanzanite in a domestic oven - so I don’t see
why not.

Helen
UK

52 
Heated to produce violet-blue color (implies that voilet blue is
the only colour that requires heat treatment which corroborates my
friend in Mt. Tsavo that mines it's description of how his
material comes out of the ground

That is a huge leap of logic to arrive at exclusionary statement,
based on loosely phrased sentence.

Leonid Surpin

53 
Recent posts regarding tanzanite stated that the occurrence of
naturally blue, unheated tanzanite was extremely rare. According
to an article in the March 2009 issue of Colored stone magazine
however, natural blue tanzanite is being found in "significant"
quantities.

If my memory serves me right, it was the rarity of faceted natural
blue that was described as rare. The phrase was “possibility of
finding one in jewelry store”. Material used for obtaining
violet-blue tanzanite is of greenish-blue hue, so the article was
kind of stating the obvious.

Leonid Surpin