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Evaluation of tanzanite

I have a query for the gemstone artists and gemological experts
among the group. I would like to ask for your professional opinions
in regards to a natural blue (unheated/untreated) tanzanite versus
the typical tanzanite which has been heated to improve the color (the
standard practice for this material). I have two questions: First,
What percentage, if any, of higher value would the naturally blue
stone have in comparison to a heated stone? Secondly, once a stone
has been cut or faceted and heat treated, is there any conclusive
test to determine that the stone is enhanced?

My appreciation in advance, for your

Michael David Sturlin, jewelry artist @Michael_David_Sturli

Michael Sturlin Studio, Scottsdale Arizona USA


All zoisite that is blue/purple has been heated. Nature provides
heat through volcanic activity or lightning strikes. The original
story of Tanzanite was that a native discovered the richly colored
blue/purple stone after lightning struck near float material. Heat
from the lightning changed the color of the stone. There was a
magnificent specimen of naturally heated zoisite on matrix that was
shown in Tucson several years ago. I think it was in the Proctor

Only material that has internal inclusions will provide evidence of
heat treating. Otherwise there is no method of detecting heat
treated In material that has inclusions you look for
minor induced fracturing around the edges of the inclusions or
discoloration of the inclusion caused by the heat. Gemologist
reports will state that the stone shows no evidence of heat

I have sold several natural stones. They were sold to “purists” who
were also metaphysacists. They claimed to be able to sense that the
stones were natural.

Value increase in these stones was minimal because they were not
super gem quality. Normally a certified natural gemstone of gem
quality will demand a price increase of 20%-50%. Sometimes more,
sometimes less.

Gerry Galarneau

all tanzanite is assumed to be heat enhanced. The market is
structured on that assumption. I know of no definitive test for
heat treatment.

Richard W. Wise, G.G.

Hi Michael, You might try contacting the GIA for specifics. Off the
cuff, I remember that tanzanite occurs in gray, brown, violet, blue,
reddish-purple, and green. The natural stone is strongly pleochroic.(
changes from reddish purple to blue to deep purple). After heat
treating (620C), the reddish purple stones are altered to violet-blue
or a deep sapphire blue. These stones have a reduced pleochroism.
Personally, I like the brown zoisites, even if there isn’t much of a
market for them. Will Estavillo

Actually tanzanite is trichroic. If you have a piece of rough and
rotate it at a certain point you will even see an orange color. I
was fortunate to be given a small parcel of rough before tanzanite
became all the rage. From one of the pieces I faceted a square
brilliant with a finished weight of just over 7 cts. It is a lovely
blue. I cut another smaller stone which is a purple blue; the piece
was from the same parcel. As it turned out some of the parcel turned
out to be iolite which also cut into a beautiful color. The sq
brilliant color is the color I would look for in a sapphire. I
think, but I’m not certain, that there is only one other stone
that is trichroic.

Many thanks to all of the individuals who so kindly replied to my
question about tanzanite. The stone in question is a beautiful
natural blue uncut crystal, which will be able to remain untreated
after cutting. From the I received, the consensus
indicates that less than 2% of this material comes out of the ground
naturally blue in color. This crystal is strongly pleochroic, when
viewed through one direction it is a lovely blue, turned 90 degrees
it is strongly violet.

It should yield a beautiful finished gem, which will be a perfect
stone for this particular client. It was a fortunate situation where
the client was able to see and handle the rough crystal before the
cutting begins, and in addition both the source and the stonecutter
can attest to the fact that the color is not enhanced.

I really appreciate the help from the many Orchid members who
contributed, both on and offline, thanks again!

Michael David Sturlin, jewelry artist @Michael_David_Sturli

Michael Sturlin Studio, Scottsdale Arizona USA

Michael & All, Did you see the Tanzanite mined out of the rock?
Unless you were there when the tanzanite was discovered there is no
way that you can say that the stone has not been heat enhanced by
man. This stone needs to be certified. I have seen many rough
crystals that have been heat treated and a lot of rough that has not
been treated. None of the rough I have seen has been strongly
blue/purple without man enhanced heat treating. This observation is
based upon viewing kilo’s of rough. Miners have learned many tricks
to fool buyers. At the mine sites miners are known to carry treated
material into the mine to mix with the days finds. I would send the
crystal to GIA, AGTA, or Capp Beasley to get a no heat
certification. A large, natural, tanzanite of strong color is very
valuable. Get it certified before cutting.

Gerry Galarneau

Back in the old days in E. Africa some of the early material found
on or near the surface was in fact quite blue. Speculation is that
over time, the sun did the job we do in an oven.

I had one stone sold to me by Dr. Barot that he said was natural, it
was also top gem.


Howdy Gerry and list, I must agree with Gerry here, not only is it
reasonable to assume fraud will take place wherever a strong enough
incentive exists, it THRIVES in situations where detection is
unlikely or carries little penalty. I have read of synthetic ruby
(lab corundum) being heated,fractured,soaked in ferric
solutions,fused,repeat process and then mixed into mine rough AT THE
MINE in SE Asia. In these small sizes it would be alomost impossible
for the average gemologist to detect in addition to the reduced
incentive to have smaller stones even checked at all. While I always
pass on to my customers all info I have on the stones I facet, people
do have weird feelings about some enhancements. If it’s toxic or
reduces the ‘lifespan’ of the stone as presented (ie. fades with time
or chages color) then that should DEFINITELY be revealed to and fully
understood by the customer. Of course a few stones are NATURALLY
subject to fading. So much other enhancement is either duplicating
what SOMETIMES occurs in the ground anyway (heat, background
radiation) or IS A FUNCTION OF CREATING A GEMSTONE that people
should learn to live with it. After all, it starts being ‘enhanced’
(made more ‘acceptable’) the moment the mud is knocked off the xtal!
Not to mention cobbing, sawing, preforming, cutting and polishing! So
what if some heat, radiation (given the subsequent safety of the
piece afterwrds) or gasses were also inserted into the processing.
This of course is not to say overgrowth or assembled stones or other
obvious attempt at fraud are acceptable. It’s my understanding that
the original type specimen for Tanzanite (1964?) (heat treated
zoisite) was belived to have been blu due to having a brush fire
passed over it. Now, does it make a difference if the fire was
started by lightning or a careless cigarrette butt? Every piece of
rough I’ve ever cut has been GREATLY enhanced!. Carl
1 Lucky Texan