December 14-16—SAN DIEGO, CALIFORNIA: Wholesale and retail show; Gem Faire Inc.; Scottish Rite Center, 1895 Camino del Rio S; Fri. 12-6, Sat. 10-6, Sun. 10-5
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PostPosted: Thu May 14, 2009 3:59 am 
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And speaking of graphs... Tim, I was looking at your learning curve again. Looks good over the range between "two days ago" and "today", but "tomorrow" seems quite a shocker.


It is... Vaccuuming, stuffing bags, dealing with a stressed out girlfriend, sitting in a car for about 2 hours, waiting to board a plane for about two hours, sitting in that plane for 2 hours and then finally that curve will go up again!

:D


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PostPosted: Thu May 14, 2009 4:09 am 
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Tim wrote:
and then finally that curve will go up again!


for sure i'll do my best for this... :D 8)
ciao
albé

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PostPosted: Wed May 20, 2009 3:11 pm 
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Pete sent me some nice pieces of Namanga garnet to have a look at. The pieces were ideal for my spectroscopy, with flat parallel sides... thanks Pete! He reports that these garnets are a pyrope/almandine/rhodolite/spessartine mix. Everything but the kitchen sink, apparently.

The color of these garnets was really interesting. Thin pieces, on the order of 2-3 mm thick tended to look orange in sunlight, while thicker pieces look deep red. It was interesting trying to decipher this color. I could stack two thin orange pieces together, and the result looked red. And I could usually angle a thicker piece in sunlight and see some orange.

Then I found one triangular-shaped piece with varying thickness that always looked red, maybe even with a pink hue. I thought to myself "this is the one that doesn't belong... this is the one that is different from all the rest."

So I decided to collect spectra from that triangular piece, a particularly thick piece (maybe 6 mm) and a thin piece (3 mm). Interestingly, the spectra themselves didn't do much to solve this color change mystery. But the very process of collecting the spectra did give me a handle.

First the transmission spectra... The black curve is the triangular piece that I thought must be different. The blue curve is the thick piece, and the red curve is the thin piece.

Image

Looking at the relative spectra, there is not much difference at all between the three samples. The transmission spectra produced by these garnets are pretty similar to the almandine spectra. I kind of thought this might be the case when colorless pyrope was listed first, and almandine was listed second. The major difference between the two materials seems to be that these garnets suppress transmitted light over the range between 400-600 nm more than does the almandine examples. So the common almandine lines that show up in the spectroscope are going to be more difficult to see with this material... i.e., it is going to look a lot more black across the 400-600 nm range.

Note that the triangular piece's spectrum looks almost identical to the thick piece's spectrum. The spectrum of the one piece I thought was most different turns out to look the same as that of another piece.

So what could be the cause of this color change. Here was the clue: when I collected the spectrum of the triangular piece, I had to integrate the light signal about twenty times longer than I did for the thick piece. About forty times longer than I did for the thin piece. So... not nearly as much light was passing through the triangular piece. So my guess is that the less light returned from the material to the eye, the more the color of the material shifts toward the red.

This material poses an interesting challenge to the cutters... do you want to maximize light return and get orange, do you want to find some happy place where different angles give different colors, or do you want to find a light return that gives red but isn't too dark. Interesting.

Thanks again Pete, this is nifty material.


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 Post subject: Just thanks...
PostPosted: Thu Nov 12, 2009 2:12 pm 
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I wanted to just say "thanks" for posting all this information, particularly the raw transmission spectra, for these garnets. Garnets are my second favorite gem (mineral family), right after opal.

And I would be remiss if I didn't also say "thanks!!" to Dr. Hanneman, too for all his contributions to our understanding of this group of minerals used as gem stones.

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 Post subject: Re: garnets
PostPosted: Tue Jul 06, 2010 5:12 pm 
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Ok, it seems that the green grossulars come in two varieties... those referred to as merelani mint and those referred to as tsavorite. Maybe these define the ends of a saturation spectrum. But over time I've seen that merelani mints tend to fluoresce and the tsavorites tend not to fluoresce. Somewhere on these boards I posted the fluorescence spectrum of a merelani mint. This fluorescence allegedly arises from chromium, which is also the garnet's alleged chromophore. In contrast, the tsavorite's alleged chromophore, vanadium, does not fluoresce.

I collected transmission spectra from green garnets, one that does fluoresce and one that doesn't...

Attachment:
green garnet.JPG
green garnet.JPG [ 30.55 KiB | Viewed 2869 times ]


The wavelength scale extends a bit beyond the visible into the infrared to 800 nm, in order to show the structure in the fluorescing garnet's spectrum around 700 nm. These narrow absorptions at 700 nm are reminiscent of similar structure in the chrome pyrope's spectrum back on page 1 of this thread. There is no observable narrow structure in the non-fluorescing garnet's visible spectrum; no narrow absorptions that can be assigned to vanadium.

It is interesting to see that the green peaks in the transmission spectra occur at two different values... about 525 nm for the tsavorite and about 550 nm for the mint.


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 Post subject: Re: garnets
PostPosted: Wed Jul 07, 2010 11:22 am 
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Now what if you were looking through the spectroscope at the merelani mint and tsavorite described in the previous post? Personally, I don't see any difference. The minimum transmissions appear at about the same places in both, 450 nm and 600 nm, so that the broad absorption bands look much the same for each. Maybe someone with a real keen eye can detect the small structure at the 700 nm edge, but I can't. Try it out for yourself... if you have a merelani mint, look at it through the spectroscope... can you see one or two bright lines right at the very edge of the red?


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 Post subject: Re: garnets
PostPosted: Thu Jul 08, 2010 8:03 am 
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fortunately the distinction between mint and tsavs is easily made by the naked eye :D


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 Post subject: Re: garnets
PostPosted: Thu Jul 08, 2010 11:02 am 
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Yes, but all mints may not be colored by chromium, or all tsavs colored by vanadium. However, it is easy to determine which elemental ion causes the color without resorting to the spectroscope. Just use one of those 400 nm UV-LEDs or 405 nm lasers to see if it fluoresces orange... if so, chromium... if not, vanadium.


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 Post subject: Re: garnets
PostPosted: Thu Jul 08, 2010 1:37 pm 
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Tim wrote:
fortunately the distinction between mint and tsavs is easily made by the naked eye :D

Isn't that pretty subjective Tim? Do you have images that show what your eye tells you? :? It seems to me it's the old ruby/pink sapphire problem redux.

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 Post subject: Re: garnets
PostPosted: Thu Jul 08, 2010 1:42 pm 
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ROM wrote:
Tim wrote:
fortunately the distinction between mint and tsavs is easily made by the naked eye :D

Isn't that pretty subjective Tim? Do you have images that show what your eye tells you? :? It seems to me it's the old ruby/pink sapphire problem redux.


Or as I usually describe it "If you're selling it's Ruby. If you're buying it's Pink Sapphire." :D

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 Post subject: Re: garnets
PostPosted: Thu Jul 08, 2010 10:17 pm 
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Brian wrote:
Ok, it seems that the green grossulars come in two varieties... those referred to as merelani mint and those referred to as tsavorite. Maybe these define the ends of a saturation spectrum. But over time I've seen that merelani mints tend to fluoresce and the tsavorites tend not to fluoresce. Somewhere on these boards I posted the fluorescence spectrum of a merelani mint. This fluorescence allegedly arises from chromium, which is also the garnet's alleged chromophore. In contrast, the tsavorite's alleged chromophore, vanadium, does not fluoresce.


i have wondered if the merelani mint garnet and tsavorite found in tanzania were "different" from the tsavorite found in kenya-chemical composition/coloring/vanadium vs chromium. (?)


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 Post subject: Re: garnets
PostPosted: Thu Jul 08, 2010 11:47 pm 
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Just to be complete, I dug up the thread where I posted the mint's fluorescence spectrum. And also the thread that got me to thinking about separating one kind from another.

In the first thread, I mention hearing that there might be an absorption line at 425 nm indicative of vanadium. But I see no evidence for it in the non-fluorescing garnet's spectrum. So in this first attempt at looking for a narrow absorption feature in a stone's spectrum that can be assigned to vanadium, I've come up empty. I wonder if Jean-Marie ever got that vanadium emerald he was looking for?


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 Post subject: Re: garnets
PostPosted: Fri Jul 09, 2010 2:35 am 
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The two green grossulars have derived their trade names from their locality and do specify certain colors. Deep, drop dead gorgeous green for tsavorite and a light 'minty' green for Merelani mint. Rick, I see your point, there will be a vast overlap between saturated mints and not so saturated tsavs.

Now... if we demand separation methods and start appointing elemental characteristics to it here after investigating two samples we'll get in the same kind of shit as chromium/vanadium emerald, copper in tourmaline and more of those old tradenames gone bad due to elemental properties that haven't been taken into account by the inventors of those tradenames.

There IS tsavorites that appear red under 405nm excitation (or through the chelsea filter). There WILL BE Merelani green ones that don't...

It's tradenames... they aren't invented to scientifically separate one garnet from the other... they have been invented to sell rocks from a certain locality.

It is mighty interesting to see Brian's work and learn from it. We just have to be very careful with our conclusions... :D


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 Post subject: Re: garnets
PostPosted: Fri Jul 09, 2010 11:42 am 
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Tim wrote:
It's tradenames... they aren't invented to scientifically separate one garnet from the other... they have been invented to sell rocks from a certain locality.

It is mighty interesting to see Brian's work and learn from it. We just have to be very careful with our conclusions... :D

We're in total agreement on both of those points Tim =D>

In the end they're all green (or some variation thereof) grossular garnets. But the trade names lead us into all sorts of strange byways on the selling end.

John D. Rouse in his book "Garnet" (1986) recognized the problem 25 years ago, saying "The green stones are now classified as either green grossular or tsavorite, depending on the intensity and tone of the green colour, very much like the arbitrary division between green beryl and emerald, or between pink sapphire and ruby." The irony in Rouse's comment is that tsavorite itself is a trade name invented by Tiffany & Co. to "sell rocks from a certain locality."

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 Post subject: Re: garnets
PostPosted: Fri Jul 09, 2010 11:47 am 
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ROM wrote:
Tim wrote:
The irony in Rouse's comment is that tsavorite itself is a trade name invented by Tiffany & Co. to "sell rocks from a certain locality."


maybe it would be good to remember the great and unforgotten Gemologist Campbell R. Bridges who discovered it in 1967... :wink:

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