Some garnets can have RIs over 1.80. This includes garnets of the andradite variety which would include topazolite. This article contains a table that compares the RIs, colors, and spectra of some garnet species and blends.
thanks for the professional information… am I correcting in saying that andradite, grossular and uvaroite belong the ugrandite family of calcium garnets with Al, Fe, and Cr substitution for each other. Andradite would be the Fe3+ end member grossular being the Al+3 end member. None of them occur as a pure endmembers. I also have read that there’s some 2+ My question is how close to the andradite endmember is topazolite? some scholarly sources show limited divalent ion substitution with the pyralspite group. Mg+2 has an ionic radius that is too small. The transition metals following Ca+2 have similar sized radii…but ae not in the divalent site…I would also presume that a high Fe content would give a garnet a higher RI and SpG. Am I correct or not?.. or it it more complicated than that?.
Hi again I have picture of the yellow gem above a thin pencil line. Sorry for the poor quality I am having trouble with a new digital mic
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whatever you have is an attractive stone. Could be quite valuable if it can be definitively ID’s as topazolite. Good luck
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Just from the look, I would guess Sphene.
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Hi thanks for the response, even if it is a flawless 7 carat sphene I would be very happy. It has been kicking around my collection loose for a long time with no visible damage. I am snowbirding at present but I will check the stone with my very sensitive Geiger counter when I return home.
Thanks again
unless there’s a lot of thorium substituting for calcium in titanite, it might not be highly radioactive. titianite is mildly radioactive at best. what is the specific gravity?
I would look for doubling, if it doubles, it is most likely zircon.
Have you considered sphene (titanite)?
I agree with the people who said sphene (titanite). A little brighter light would help, as a clean, well cut sphene tends to disperse multi-color like a diamond. I have one in a pinky ring and people always comment on it and want to know what it is.
I think the high dispersion is more interesting than the R.I… garnet (even topazolite) does not have that high of dispersion… you have a real rainbow of colors coming out of the gem. Now… I have cut some “antique glass”… old Pb-glass that is exactly the same color and looks very similar (it is suppose to have a high RI value… I haven’t measure mine for same reason… my refractometer doesn’t go above about 1.80… but it does have the righ dispersion and the right brilliance. I you put it under a polariscope (or just between two sheets of polarized films… and it stays dark as you rotate… then it is probably antique glass … garnet would do the same as it is isometric… both sphene and zircon will “blink” light and dark every 90-degrees of rotation. That might help with the ID.
It’s either a yellow zircon (probably), sphene or sphalerite IMO. Is it doubly refractive? If so, definitely not a garnet, which is single.
This looks like Sphene to me. If it is sphene you are lucky to have such a large size. It’s beautiful.
Hello again and thanks for all your replies. I look carefully but I do not see any doubling so as someone mentioned do a specific gravity test so I did multiple times just using a three place balance and came up with 3.795 which maybe Andradite Garnet. I am not holding my breath yet but it looks exciting
Lou
Hello again I did some further test using my gem eye meter and tested three stones the garnet a zir on and the yellow stone. Yellow stones first zircon next then garnet.
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Well just received my Presidium RI tester and was eager to test my sparkly yellow gemstone. Am I rich?.
No the refractive index read 2.17 Cubic Zirconium oh well still a pretty stone.
Thanks for all your input.
Lou
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Lou,
It seems that you have found an answer on the stone. It definitely has a few interesting measurements to its name now… 
I have some questions/comments on the instruments you have, if you don’t mind talking about them here.
Spectrometer:
- What spectrometer are you using? You have some interesting spectra on that stone.
- Have you done any tests on other stones to characterize its output? (Like the Garnet and Zircon used with the Gem-Eye)
The Gem-Eye tester:
The instrument uses thermal conductivity. Using it on stones in direct sunlight will affect its measurement. What is the blue putty material? That could also affect the measurement, by generating additional thermal mass especially in direct sunlight.
Admittedly, it did make a handy ‘stone holder’, though…
Cheers!
-Troy
Hi thanks for your reply. I like testing different spectrometers first is the thunder optics (quite inexpensive) used with spectragryph software second gospectro which attaches to a smart phone and linksquare which again is used with a smart phone. I am hoping to purchase a Raman spec sometime if I get a buy on a used handheld unit. The gem-eye works well in a cool room and when properly calibrated, I always test known gems first but most important a cool environment, sun not too important. The blue tack material is the material used in watchmaking or the stuff they sell for mounting pictures on the wall, never found it to be themally conductive. Hope that answers any questions.
Lou
Further to your questions I run spectra on known gem stones and then compare them to the unknown if the spectra superimpose then I am fairly certain that they may match. Color difference may show a slight change in specta. I am looking at Raman spectrometers used for drug testing and explosive testing at borders or by fire department etc. Spectra produced are the gold standard and very reproducible.
Lou,
Thanks for the quick response! I thought the blue stuff might be rodico but was thrown off by the deep blue color… I have only used/seen the sea-green type from Bergeon.
I was curious about the spectrometer types, so thank you! I have been searching for and looking at several types.
I totally agree, a Raman is in my bucket list, too. Although, I don’t think I have come across a hand-held model… seems more of a desktop item as they are bulky to feed the power-hungry laser module.
If you have seen any models… would be interested in looking around… I have access to several test equipment vendors.
Troy