I always wonder if BLUE garnet is sitting among Spinel.
Refractive index (R.I) of garnet family varies from 1.69 to 1.89.
More specifically R.I of Grossular garnet is 1.69 to 1.73, within
the R.I of Spinel which is 1.71 to 1.73. Garnet and Spinel being
isometric, whoever who has basic testing tools (polariscope,
refractometer etc) trends to conclude that any blue singly
refractive stone within this range is Spinel. Because blue spinel is
common and no known blue garnet.
Is there any other technique to identify without x-raying.
Garnet and Spinel being isometric, whoever who has basic testing
tools (polariscope, refractometer etc) trends to conclude that any
blue singly refractive stone within this range is Spinel. Because
blue spinel is common and no known blue garnet.
I have been wondering about it myself. It is quite possible, but
there is no way to prove it until someone finds a blue garnet. As far
as identification goes, they distinct in density, hardness, spectrum,
and etc. None of if is a definite proof of identity, but combination
of them is a very good indication. To confirm if the stone is indeed a
blue garnet, would indeed require sophisticated testing.
There is no blue garnet. I think the GIA and most other gem labs
around the world have pretty much established that fact. You can’t
rewrite the science books just because you want to.
Spectroscope might be your key to separation between the two. Of
course internal characteristics will tell the tale. There is no known
blue color in the Grossular Garnets. More often reddish orange
grossular might be confused with similar colored Spinels. The RI
though is still a good guide. Yes Synthetic Spinel does have an RI
range that can overlap with Grossular Garnet. Natural Spinel is
unlikely to overlap. Syn. Spinel is likely to fluoresce more than
Garnet and the Garnet will often contain natural inclusions (and will
have a more natural looking color). Good magnification combined with
RI, fluorescence and maybe spectra should do it. Interesting area of
gem separation.
Now there are some rare color change and color shift Garnets that are
various shades of blue and turn to a shade of purple. Have seen some
from Madagascar. They contain Vanadium. I think they would fall into
a higher RI range though. They would also have natural inclusions.
Any garnet with a bluish coloration or tinge is a
color-changer…shifts from bluish to purplish, much like alex.
Almost any blue spinel will be rather clean…all the bluish
garnets I’ve ever seen are full of inclsuions…and have that
"garnety" extinction problem due to high Mg/Fe levels. The two just
don’t look anything alike at all.
There is no blue garnet. I think the GIA and most other gem labs
around the world have pretty much established that fact. You can't
rewrite the science books just because you want to.
Read the story of how Taaffeite was discovered. I do not have a
link, but it should not be much trouble to find it on the Net.
GIA had a garnet symposiun recently and among the color topics
talked about was the discovery of apprantly blue garnet...
Apparently my age and the dating of my gemological training is
showing a bit. Hadn’t heard about the GIA garnet symposium and the
discovery of blue garnet. Perhaps I really do have to catch up on all
those issues of Gems and Gemology I can’t ever seem to find the time
to read.
Daniel R. Spirer, G.G.
Daniel R. Spirer Jewelers, LLC
Now there are some rare color change and color shift Garnets that are
various shades of blue and turn to a shade of purple. Have seen some
from Madagascar. They contain Vanadium. I think they would fall into
a higher RI range though. They would also have natural inclusions.
Indeed, and not cheap either. There is some for sale at hassamventures.com under the garnet section. $1500 for a six carat
rough stone. The picture looks pretty blue to me.
I always wonder if BLUE garnet is sitting among Spinel.
Refractive index (R.I) of garnet family varies from 1.69 to 1.89.
More specifically R.I of Grossular garnet is 1.69 to 1.73, within
the R.I of Spinel which is 1.71 to 1.73. Garnet and Spinel being
isometric, whoever who has basic testing tools (polariscope,
refractometer etc) trends to conclude that any blue singly
refractive stone within this range is Spinel. Because blue spinel is
common and no known blue garnet.
Is there any other technique to identify without x-raying.
KAD-- Spectroscopy should separate garnet from spinel. You can get a
hand held spectroscope pretty inexpensively then all you have to do
is figure out how to wrestle the darn thing into submission so you
can read the spectrum of your sample. Very tricky to get the light
just right with a handheld, but with practice, practice, practice,
you can get a decent spectrum on many stones.
If you’re flush, the prism based units with stand, stone holder and
lighting are much easier to use. Once you develop skills and
confidence in using a good spectroscope, it’s one of the quickest and
most sure tests for many gem identifications.