Strange CZ?

I bought two pieces of CZ from a supplier recently, one garnet color and the
other emerald green. The difference in weight between the two pieces was
way out of proportion to the difference in size so I did an SG on them and
got 4.2 for the red and 2.7 for the green.

I checked the RI of the green piece and came up with 1.8.

After several email exchanges, the supplier said: "you won't believe this
but it is CZ"....

Should I believe it?

The SRB I cut from it is a dazzler, briliant and nice fire for a green
stone. I am not unhappy with it but I would really like to know what it is.

js
--
Astronomy, Beer, Cheese, Gems, Sausage http://schmidling.netfirms.com

"Jack Schmidling"
I bought two pieces of CZ from a supplier recently, one garnet color and the
other emerald green. The difference in weight between the two pieces was
way out of proportion to the difference in size so I did an SG on them and
got 4.2 for the red and 2.7 for the green.

I checked the RI of the green piece and came up with 1.8.


That's strange. From the SG you determined neither of the two stones could be
CZ.
CZ has an SG of 5.6-5.7 for the Calcium-stabilized variety, and 5.7-5.9 if it
contains Yttrium.

Are you sure of the RI=1.80?
Normally, fresh contact fluid has an RI of 1.81, but when it is cold a lot of
sulpur precipitates
out, and the RI is lowered.
So it could be possible that the real RI of the green stone is greater than
1.81 and that the
dark-light region that you read on the refractormeter scale is actually the
position of the RI of
the contact fluid.

The funny thing is that CZ is not too expensive, so I can't see why a supplier
would "swindle" you
by selling you something else.
The only candidate would then be would glass, which can have SG between 2.3-4.5
and RI between
1.44-1.90.
So perhaps looking for air-bubbles with a loupe or microscope could be an
option?



Groeten,


Jaap

"Jaap Bos"

That's strange. From the SG you determined neither of the two stones could
be
CZ......

Got some egg to wipe off my face.

I forgot to take the sample basket into consideration when I did the SG so
the numbers are much too low. However, a large difference still exists
between the two materials.

The samples are both about 35 grams so my measurement error is not a serious
part of the difference.

The corrected numbers are green=4.7 and the red =6.0

The RI are still green=1.85 and red =2.22

I am measuring the RI with a microscope so all the comments about liquid do
not apply.

Looking at the numbers now, it appears that the red is CZ and the green is
probably YAG.

I just finished cutting an SRB from the red and although very bright and
glitzy, all the fire is red. Not surprising except for the fact that the SRB
cut from the green has lots of fire of the usual rainbow colors. How this
can be from a green stone is a puzzle but the difference is obvious.

In light of this, I am in no hurry to return it for credit.

The funny thing is that CZ is not too expensive, so I can't see why a
supplier
would "swindle" you by selling you something else.

I never gave that a thought but it is easy to imagine getting rough samples
mixedup or mismarked either by the vendor or his supplier.

js

--
Astronomy, Beer, Cheese, Gems, Sausage http://schmidling.netfirms.com

On Wed, 26 May 2004 22:18:55 -0700, in rec.crafts.jewelry Jack Schmidling
wrote:

I just finished cutting an SRB from the red and although very bright and
glitzy, all the fire is red. Not surprising except for the fact that the SRB
cut from the green has lots of fire of the usual rainbow colors. How this
can be from a green stone is a puzzle but the difference is obvious.

While I wouldn't be surprised to see a red CZ not showing a lot of dispersion
in other colors, I WOULD be surprised to see a YAG showing unusual mounts of
dispersion at all. It's not noted for high dispersion. A lot lower than C.Z.

Now, I'm no expert at determining R.I with a microscope, but if memory serves,
it's not that easy to get high accuracy. How sure of that figure are you? by
the way, did you check the behavior of these stones in a polariscope?

Peter

"Peter W. Rowe"

Now, I'm no expert at determining R.I with a microscope, but if memory
serves,
it's not that easy to get high accuracy. How sure of that figure are you?

The absolute number is subject to the ability to determine best focus
through the stone and I put that at around 10% or better. Considereing the
diference I see, the margin of error is not an issue in this case.
Furthermore, I get the same numbers (within a few %) everytime I measure
them sothe repeatability is very good with a little practice.

by the way, did you check the behavior of these stones in a polariscope?

Don't own one.

js


--
Astronomy, Beer, Cheese, Gems, Sausage http://schmidling.netfirms.com

"Peter W. Rowe" pwrowe@ixDOTnetcomDOTcom
Now, I'm no expert at determining R.I with a microscope, but if memory serves,
it's not that easy to get high accuracy. How sure of that figure are you?

I don't know if Jack tested this method on stones of know RI, but my experience
is that if you take
a number of precautions the accuracy of the microscope method is not too bad!
Using high
magnification (x80), measuring the true size of a stone to 0.01 mm., measuring
the "optical depth"
of the stone 6 times and averaging the result, then for a number of
brilliant-cut, 200-800 mg
stones ( like GGG, Anglesite, Strontium Titanate, CZ, YAG) I never deviated
more than 0.12 from the
true RI (and for some unknown reason my results always were lower than the true
RI).
For stones with an RI above the range of the gemmological refractometer this
microscope result,
combined of course with an SG-determination, is nearly always accurate enough
for a positive
identification (keeping my fingers crossed) .

Groeten,


Jaap

"Jack Schmidling"

The corrected numbers are green=4.7 and the red =6.0

The RI are still green=1.85 and red =2.22

Looking at the numbers now, it appears that the red is CZ and the green is
probably YAG.

With SG=4.7 and RI=1.85 I can't think of another possibility.
At least not found in my collection of synthetics, that contains a lot of weird
(laser-crystal)
stuff like neodymium-gallium-garnet, samarium-gallium-garnet, YAP, YAP with a
lot of thulium, etc,
etc).

So I think you did a good buy, YAG instead of CZ.


Groeten,



Jaap

"Jaap Bos"

I don't know if Jack tested this method on stones of know RI, but my
experience
is that if you take a number of precautions the accuracy of the microscope
method is not too bad!

What I have learned since using this technique is that the accuracy is
absolute by definition but the repeatability or uncertainty is the problem.
This is strictly a function of deciding the point of best focus. I find
that using 30X, this is about .005". Obvioulsy, the thicker the sample, the
less significant the error becomes.

It also makes no sense to measure the real height of the stone this way
because you unnecessarily insert the same error again. An inexpensive
caliper will provide much more accruate measurements, easily .0005" which
for all practical purposes, can be ignored.

I don't have a refractometer so it came as a surprise to learn from others
that they are not very accurate at high RI. Not sure why this would be but
the microscope technique does not suffer from this problem. As long as
one surface is polished and you can see through the sample, it is as
accurate as your estimate of best focus. Taking several readings and
averaging them is always a good idea.

I have tried higher powers but don't find that it improves things much so
30X seems a reasonable compromise.

It has also been suggested that one needs a dial indicator with .001 mm
resolution but this is nonsense as the best focus point is still just as
subjective and anything over .001" is overkill.

js


--
Astronomy, Beer, Cheese, Gems, Sausage http://schmidling.netfirms.com

On Sat, 29 May 2004 08:00:20 -0700, in rec.crafts.jewelry Jack Schmidling
wrote:


"Jaap Bos"

I don't know if Jack tested this method on stones of know RI, but my
experience
is that if you take a number of precautions the accuracy of the microscope
method is not too bad!

What I have learned since using this technique is that the accuracy is
absolute by definition but the repeatability or uncertainty is the problem.
This is strictly a function of deciding the point of best focus. I find
that using 30X, this is about .005". Obvioulsy, the thicker the sample, the
less significant the error becomes.

the accuracy one can achieve also seems, at least for me, to depend some on the
eyes of the examiner, as well as individual microscopes. In my own case, both
my scopes have rather still rack and pinion mechanisms. It makes very tine
adjustments in focal point slightly more likely to introduce a tad of
vibration, which can shift the stone slightly in the stone holder. Obviously,
there are fixes for this (more rigid stone holders, lubricating the rack and
pinion, etc), but it points out that there may be possibilibies for error that
go unnoticed. Also, in my own case, my two eyes have significantly different
refraction. I'm used to using a microscope without glasses, and adjusting the
two eyepieces to compensate for the difference in my two eyes. It seems to
work well enough, but even so, I often find that finding the exact point where
the focus in one eye matches that in the other, is tricky, since my eyes tend
to compensate somewhat already. Normally, this means the adjustment isn't
critical for me. but when measuring RI with a scope, it makes it more
difficult, since if the two eyepieces don't exactly match my eyes, and i've
simply compensated, then attempting to find an exact point of perfect focus is
harder, as there is, in essence, a deeper depth of field, since the two eye's
have overlapped but not matched depths of field. Nevertheless, with larger
stone sizes such as we're talking about here, it can be reasonably useful.
Most of the stones I've needed to try this with have been much smaller, more
along the carat size range, and in those cases, I've found the error range, at
least for me, to be too great to give more than a general indication of the
RI's probably range, and ordinary visual observation of the stone usually also
can do that pretty well.


I don't have a refractometer so it came as a surprise to learn from others
that they are not very accurate at high RI. Not sure why this would be

I suspect that you may have mis interpreted what happens with refractometers.
I've never heard they are less accurate at higher RI readings within their
range. However, what IS the case is that they are only good up to an RI of
about 1.81 with normal equipment. They are limited in the range to which they
can read by needing an optical contact fluid to eliminate the air between the
hemisphere and the stone, as well as being limited by the RI of the hemisphere
itself. With the usually available equipment, this sets the maximum RI they
can read at about 1.80 (a stone reading 1.81 cannot be distinguished with
certainty from the normal high reading of 1.81 that the instrument gives just
with no stone at all, or with stones higher than 1.81), and stones up to that
point can be measured with full accuracy of the instrument, while stones higher
than that point cannot be read at all.

Peter

"Peter W. Rowe" pwrowe@ixDOTnetcomDOTcom

Also, in my own case, my two eyes have significantly different
refraction. I'm used to using a microscope without glasses, and adjusting the
two eyepieces to compensate for the difference in my two eyes. It seems to
work well enough, but even so, I often find that finding the exact point where
the focus in one eye matches that in the other, is tricky, since my eyes tend
to compensate somewhat already.

Yes Peter, I see your point.
I'm the proud owner of :
one nearsighted eye
one farsighted eye

(for the rest I'm doing quite well!!)

So in this technique I always take readings using only one (my best?) eye.
That will remove some of your problems.

Most of the stones I've needed to try this with have been much smaller, more
along the carat size range, and in those cases, I've found the error range, at
least for me, to be too great to give more than a general indication of the
RI's probably range, and ordinary visual observation of the stone usually also
can do that pretty well.

Yes, one carat is a bit small for this technique
In an earlier posting I said that I deviated not more than 0.12 RI-units.
But that was an extreme value, normally the difference is not more than 0.06
RI-units.
(is a bit in line with what Jack posted, although he presented his results in
inches, but that is
about the same order of magnitude of my measurements).
So I still think, that if you try this technique (first with large stones of
known RI) you will end
up estimating even one carat stones with a more or less reliable RI.(very
impotant for minerals
with an RI1.81, since even reflectance meters like the Sarasota Digital
JMeter-90 are not that
precise)

One more question. When you say:

and ordinary visual observation of the stone usually also
can do that pretty well.

Do you look at the stone and say "well the lustre of the stone is
sub-adamantine, so the RI should
be high"
or do you use "visual optics" which also can give an estimate of the RI of
brilliant-cut stones?