Strange CZ?

On Sat, 29 May 2004 15:17:02 -0700, in rec.crafts.jewelry "Jaap Bos"
wrote:

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?

Visually examining the stone incorporates lots of combined impressions,
including the nature of the luster, the polish, facet rounding, condition of
the girdle, and what you can see of R.I. related effects as well as
dispersion. These usually leave a beginning impression of R.I., which can then
be elaborated on with other more specific observations. Luster itself is one I
don't usually conciously use, as it's so dependent also on polishing technique
in a cut stone. Things I might do migh include looking at the read through
effect on upside down stones, both in air and in liquid, as well as looking to
see how far I need to tip a stone to the side before I can find a windowing
effect looking into the pavilion.

There are people who do this sort of thing with much more precision
than I do, but I usually get enough of an indication so as to give me what I
need, especially when combined with other tests, such as polariscope, perhaps
spectra in colored stones, looking for the general degree of dispersion, and
the like. One thing that i find helps a LOT is that i've got a small
reference collection of various colorless synthetics and a few other problem
materials, that i can compare an unknown with for things like read through, or
the reflraction/reflection patterns one gets on a surface when a stone is place
table down a little above that surface in a glass container, with a light
directly overhead. It's quite distinctive, though somewhat hard to describe
verbally.

by the way, you, or someone ( I forget who, and am being too lazy to look back
and see) , said they didn't have a polariscope. they are easy to make if you
don't wish to pay the money to buy one. the main raw material is a decent pair
of old polaroid sunglasses, with lenses in the usual neutral grey color. Pop
out the lenses and see if when held at 90 degrees to each other, one over the
other, they block most light other than a faint puplish hue, from going through
both lenses (put a light bulb underneath the two) If that's the case, then
just find some jury rigged way to mount the two lenses in that position with a
gap between, where you can hold an unknown stone.

If you don't wish to use sunglass lenses, which can be admitedly a crude device
when you're done, polarizing filter material is not that costly, and is easy to
find at places like Edmund scientific, including their bargain basement
section, or american science and surplus, or other such surplus dealers. Old
photo type polarizers work fine too (so long as they are not circularly
polarized. You need linear polarizers).


The appearance of a stone examined between crossed polarizers is a very quick
way to determine whether a stone is single or double refractive, or if single
refractive, the nature of existing strain patterns. It's a quick test for
things like moissonite (which is double refractive), or glass (shows
distinctive strain pattern) or synthetic spinel (also the strain pattern is
distinctive) Often in practical use of gemmology one does not need to
fully identify an unknown stone, only to rule out some obvious choices.

This may not be the case in things like an
appraisal, where you really need to know what a stone IS, but just working at
the jewelers bench, often it's enough to rule in or out the main choices;
questions like "is this diamond, or moissonite, or sapphire/ruby/etc instead of
something else..." At the least, simple tests like that can serve as the
gatekeeper for more extensive examinations when needed.

Peter

Jaap Bos wrote:

Jaap Bos; now that name reminds me of my youth. I see you are from Holland.
Maybe your are old enough to remember the very cool Dick Bos.

The Dutch James Bond, long before there was a James Bond. Although he was not
a
secret agent, but rather a detective. We got these small paperback comic
books,
with just one image per page. They all had the same number of pages. And they
were forbidden in High School.

I wish I still had my old collection. Would probably be worth a small fortune
today.

Check it out: http://www.dickbos.com/home1024x768splash.html and
http://home01.wxs.nl/~duckburg/Dickbosstart.html

Abrasha
http://www.abrasha.com

"Peter W. Rowe"

Also, in my own case, my two eyes have significantly different
refraction.......

This test is not improved by using both eyes. In fact, you can probably get
more repeatable results using just one eye.

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.....

This was what the person I bought the CZ from said and I guess he was just
referring to the upper limit of the common refractometer which would be
useless for measuring CZ.

Funny you should mention making a polariscope. I did exactly this today. I
found a web site that described how it works so I cobbled something together
but ran out of gas when it came to understanding what to use it for. None
of the gem characteristic charts say anything about this characteristic so
maybe you can spell it out.

BTW, I just put one lens of the sunglass on the light deck and put a Petri
dish over it. The sample goes on the Petri dish and the other lens is in an
eyepiece.

Now.... what do I do with it?

js


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

On Sun, 30 May 2004 00:03:13 -0700, in rec.crafts.jewelry Jack Schmidling
wrote:


"Peter W. Rowe"

Also, in my own case, my two eyes have significantly different
refraction.......

This test is not improved by using both eyes. In fact, you can probably get
more repeatable results using just one eye.

Possibly. However, I have some difficulty seeing good focus through a
monocular microscope. It likely has something to do with the rather extensive
laser surgery i've had in both eyes to treat diabetic retinopathy. It leaves
the macular vision alone, but plays major havoc with peripheral vision,
affecting things like my ability to adjust to changes in contrast, among other
things. Plus, I'm just not as young as I once was. While I can see well
enough through the microscope, I have a lot more difficulty deciding on a
single precise point of best focus than i used to do. With both eyes doing it,
it's better than with just one. But i suspect that before that surger, or when
i was a good deal younger, that might not have been the case.


Funny you should mention making a polariscope. I did exactly this today. I
found a web site that described how it works so I cobbled something together
but ran out of gas when it came to understanding what to use it for. None
of the gem characteristic charts say anything about this characteristic so
maybe you can spell it out.

The main thing you do with a polariscope is to classify materials according to
the major optical catagories of singly refractive and doubly refractive. The
single refractive materials are those that are amorphous in structure like
glass, or which crystalize in the cubic system, such as garnet, diamond,
spinel. Double refractive materials are pretty much all the other crystal
systems, and within the double refractive stones there are several sub
catagories of uniaxial and biaxial stones, meaning those with one versus two,
optic axes, and then within these catagories, one can further seperate stones
into optically positive or negative.

Note that with careful use of a refractometer, one can make most of the same
determinations, sometimes with greater accuracy, but it's time consuming, and
of course limited to just those materials who's R.I's are low enough to measure
with a refractometer.

the basic use of a polariscope involves putting the sample between crossed
polaroids, and rotating it around the verticle axis. This is done with the
stone in several orientations, so several axis directions are thus observed. A
single refractive stone appears generally dark, or shaded, between crossed
polaroids, unless it's got significant internal strain, which can show up as
blotches of brighter lighted areas. Rotating the stone may make these areas
vary, but in a usually irregular manner. Glass tends to show a somewhat cross
shaped light figure, that doesn't move around much otr change much as you
rotate the stone. Some garnets and synthetic spinels may have so much internal
stress that the light patches and areas that show may appear like what happens
with double refracive stones, but usually, careful examination will show you
what's really going on.

Double refractive stones, when rotated between the polaroids will show a
distinct 'blinking" effect, with the whole stone, or most of it, appearing to
become lighter and lighter, then darker and darker again, lightening and
darkening with each 90 degrees of rotation. In stones with curved surfaces,
like cabs, beads, etc, which can focus light, it's more like a light ray of
light that sweeps across the stone rather than the whole stone turning light
and dark. it's important to check several orientations of the stone, since if
one happened to be looking directly along the optic axis of the stone, the
blinking won't show up. intead, a steady figure of a light cross, sometimes
with bullseye rings around the center would show up if the stone were a round
bead, or a good cabochon in the right orientation, or in most cases, if you had
a good strong magnifier or glass ball held just above the stone to resolve that
optic figure.

The polariscope is useful simply because there are so many cases where an
unknown stone might be one of several choices, some of which might be single
refractive, and others which might be double refractive. Just rotate the
sample between the crossed polarizers, and see if it stays dark, or whether it
blinks light and dark, and you've determined which class it's in. If it's
double refractive, you can also then spend a bit more time and determine if
it's uniaxial or biaxial, by using that glass ball or lens I mentioned, to
resolve the optic figure, which is different for the two types (Uniaxial shows
that cross shape with bullseye rings around the center poing, while biaxial
figures are those rings surrounding a single line, and there will be two such
bullseye with line figures that can be found at some variable angle between
each other (actually four, if you figure that each bullseye figure can also be
seen from the other side of the stone.)

While the information from a polariscope is usually one of general
classification, rather than a precise figure for a specific single stone, there
are some stones that are quite specific. Synthetic spinel, the flame fusion
variety, shows a very unique patchy/fibrous sort of strain pattern that's not
resembled by most other stones. Glass too, is pretty distinctive, and the
interference figure shown by quartz (when it's twinned, which is almost always
the case with natural quartz) shows an unusual anomaly that almost prooves the
stone to be quartz, as very few other stones do the same.

Additionally, one can use the polariscope to examine colored double refractive
stones for pleochroism, and examine the different pleochroic colors visible,
as well as determining the directions in which individual colors are the
strongest. This can sometimes be of use to gem cutters, in choosing the best
orientation for cutting. Likewise of use to cutters, but also to setters,
stones can be examined for internal strain, which might put a stone at greater
risk of breakage during cutting or setting. Strain shows up as anomolous
bright areas under the polariscope that are not part of the blinking effect or
optic figure.

I suggest you buy, or borrow, almost any good book on gem identification.
They'll all give you instruction on the use of a polariscope, hopefully with
decent photos of the various optic figures, which may help you more than my
poor verbal descriptions can do.

and if you want to take a break and see some pretty colors, examine some
cellophane plastic wrap (like the wrapper on a cigarette package, or the like)
though it. Rainbows galore... many plastics do that, and it's interesting to
watch the patterns change as the plastic is deformed as well.

cheers

peter

BTW, I just put one lens of the sunglass on the light deck and put a Petri
dish over it. The sample goes on the Petri dish and the other lens is in an
eyepiece.

Now.... what do I do with it?

js

"Abrasha"
Jaap Bos; now that name reminds me of my youth. I see you are from Holland.
Maybe your are old enough to remember the very cool Dick Bos.


Sure, I remember him. Dick was also an adept in jiu-jitsu, a martial art that
was popular before
they invented karate.

And they were forbidden in High School.


Bad for the education, he.
There was not a lot of dialogue in the books. Mostly text like WHAMM, CRUNCH
and BOIIIING while
Dick was busy destroying enemies.

Ah, nostalgia aint what it used to be.

Jaap

"Peter W. Rowe" pwrowe@ixDOTnetcomDOTcom
On Sun, 30 May 2004 00:03:13 -0700, in rec.crafts.jewelry Jack Schmidling
wrote:

BTW, I just put one lens of the sunglass on the light deck and put a Petri
dish over it. The sample goes on the Petri dish and the other lens is in an
eyepiece.

Now.... what do I do with it?

js

One small addition to Peters text.
When you have mastered making the distinction between single and double
refraction and want to find
out if a stone is uniaxial or biaxial, it is very important to use a diffuse
lightsource. The
interference pattern will not show up if light travels through the stone in
only one direction. A
piece of frosted glass (or even a paper tissue) between your lamp and the stone
will do fine.

Groeten,


Jaap