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 Post subject: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 16, 2019 12:16 am 
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Hi,

I am new to the GemologyOnline community.

I have a question I was hoping someone might answer. I have a Tanzanite (seller guarantees it's real, no certificate) that shows light brown under a chelsea filter as it's supposed to, slightly dichroic under dichroscope, but under the Hanneman Tanzanite filter and a strong incandescent bulb, the stone remains dark blue, when it is supposed to pink-ish. I did manage to find one pink relfection of a pavillion facet, but that's all. Can this happen with Tanzanite or is it fake? I do not have a refractometer (working on it), but with my 1 ml graduated cylinders (had to estimate volume displacement between the ml lines) and some math, I found the SG to be about 3.22 - close to the 3.35.

Thanks!
Mitch


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 16, 2019 2:24 am 
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3.22 is spot on for sy thetic forsterite. Can you see eye-visible facet doubling? Any UV reaction?

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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 16, 2019 12:16 pm 
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Filters can not identify anything.

As Stephen mentioned, synthetic forsterite is often used to imitate tanzanite.
Forsterite is strongly birefringent, meaning that when viewed with magnifications the back facets will appear to double.

Tanzanite, not.


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Thu Apr 18, 2019 4:52 pm 
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Can you not find any gemologist who could take a RI for you ? That would solve the question at once.


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 23, 2019 3:22 pm 
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Thank you all very much for your replies.

The stone has no birefringence whatsoever. Under UV, the blue is more intense, but that's about it. I just recently bought a polariscope, and I do see the characteristic flashes when rotating the stone. It is just weird that the Hanneman Tanzanite filter is so off the mark.

I have to wait for my 67th birthday in June to get my refractometer (my grandkids want to get it for me). Then the stone's secret might reveal itself to me .

Does any one have any suggestions on a good refractometer? I see a lot of them on Ebay, and even Amazon, but they all seem to be the same. I am trying to find a good one with clear and accurate scale readings. Some have monochromatic yellow filter with white LED, some have yellow source light. Most use 1.80 diffraction oil. Is there that big of a difference between 1.80 and 1.81 oil, besides price?

Very confusing.

Any help would be appreciated.

Thanks Again,
Mitch


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 23, 2019 3:37 pm 
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Good, reliable instruments produce good and reliable results.
May I suggest what I use most often:
http://www.gemproducts.com/refractometer.html

And this comes from someone who also owns Eickhorst, Gem-A, and Krüss refractometers.

The limit of the refractometer is a function of the RI of the contact liquid. !.79 vs. 1.81.
1.79 is a fine and adequate place to start.


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 23, 2019 9:11 pm 
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Many thanks for the most valuable advise!

The ones I have been looking at are around $100.00 or so. I am sure it is true in this case that you get what you pay for. I will go for the one you recommended below.


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 23, 2019 9:39 pm 
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Mitch,
Since you have a polariscope you can eliminate glass, and also see the trichoric (3) colors on the different axis if it is in fact tanzanite.

Use the polariscope with the lenses uncrossed. That means in their lightest configuration rather than the darkest. Put your stone in the polariscope and rotate it. By the time you have seen it in all directions you should see three distinct colors. Sometimes two of the colors are close enough to confuse beginners, but you should easily see at least two colors.

Let is know what you see.

Steve


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 23, 2019 10:20 pm 
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-A lot of folks misinterpret anomalous double refraction for uniaxial or biaxial reactions. This is not only the case with newbies, I've seen it happen with gemmos who should know better.

-I was taught that Tanzanite, when heat treated, will be primarily dichroic as heat removes or greatly reduces the yellow green or brownish pleochroic color, maximizing the blue and violet

I am confused about the UV reaction. When you say it appears bluer, could it be a reflection of the UV light OR is the stone actually glowing?

Here is a chart we put together several years ago for the Gemology Project yo seperate tanz from its imitations.
http://gemologyproject.com/wiki/index.p ... =Tanzanite
Image


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 23, 2019 10:35 pm 
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Yes, the way I instructed him to use the polariscope basically does the same work as a dichroscope.


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Tue Apr 23, 2019 10:56 pm 
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Yep, viewing a stone using plane polarized light can also be achieved by giving a stone a spin on a laptop's screen using a white background. The light emitted from the screen is linearly polarized.
Won't work with a cell phone because they emit circular polarization.
:D

Give your unknown a spin and report back.


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Thu Apr 25, 2019 3:04 pm 
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Thanks again, everyone.

Many (many) years ago I used to facet gems and obtained a natural piece of Tanzanite from the source in Tanzania. I cut the stone for a ring for my wife. I ran this stone through all the tests I did for the other stones (as best I could while it is set in the ring), and what a difference!

(Side note: many years ago I cut a 53 ct. natural blue topaz that was displayed in the main lobby of the Denver Museum of Natural History).

This stone is pink under the Hanneman Tanzanite filter, and strongly dichroic (blue/purple), if not trichroic under the dichroscopes.

As far as the mystery gems are concerned:

1bwana1 - I did as you suggested with the polariscope lightest configuration. The color change was nothing like the stone I had cut. I could barely detect blue to a grayish-purple. However, I found a point on the polariscope about halfway between full open and full closed, that the dichroism made a profound appearance. Very noticeable between blue and purple. I thought the polariscope always had to set to full closed where the "X" appears. Apparently this is wrong.

Barbra - Thanks for the helpful table and the very kewl trick with the white screen monitor. That actually worked as well, although not to the degree as the "half-closed" polariscope.
All in all, I am beginning to believe they are heated (if not overheated) Tanzanites. I am still bothered by the lack of response to the Hanneman Tanzanite filter. I would expect it to look like the stone I originally cut from known source material.

I am so looking forward to getting that refractometer!

Thanks Again and Best to All,
Mitch


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Thu Apr 25, 2019 5:16 pm 
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Tanzanite can have some odd impurities at times. Raman scans often show broad humps above the fingerprint zone which are typical of rare earth element impurities, for instance. Even aside from that, iron manganese and chromium at least can play significant roles in the coloration of a stone. Maybe one of these is causing some odd features in the absorption spectrum? Or maybe not, a raman scan would definitely get some quick answers.

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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Thu Apr 25, 2019 5:22 pm 
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Thanks, Stephen.

Sounds expensive :)

One thing is for sure, my little hand held spectrometer is worthless.

Mitch


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 Post subject: Re: Hanneman Tanzanite Filter unexpected results
PostPosted: Thu Apr 25, 2019 6:56 pm 
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If you're near raleigh nc I would scan it for free but I suspect it is unlikely.

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