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Techniques for CollectorsHow did he do that? Diamonds vs CZ

15th Sep 2009 02:12 UTCJenna Mast

Someone recently gave me some jewelry. I'm not the biggest jewelry person in the world and typically prefer my stones in their natural form in their original matrix, but it was a gift and gave me the opportunity to get a good look at a less than microscopic diamond, along side a cubic zirconia of roughly the same size and cut.

I wasn't aware of the identities when they were first given to me. They look pretty much the same. They seemed to have the same shade, they both had sharp facets and sparkled quite nicely. The only differences I could see under close examination was one had visible flaws which gave it the effect of seeming more faceted, while the other did not. I assumed the one with the flaws was an actual diamond, and this was confirmed when I later found the tag that came with it.

I was skeptical of the flawless stone based only on the fact that such a flawless diamond of that size seemed too unusual. Out of sheer curiosity I took it to a jewelry who's family had been in the business for generations, and he was able to tell me in about five seconds looking through an eye loop in the dim light that it was cubic zirconia.

My question is, how did he do that? What is it that tipped him off?

15th Sep 2009 05:25 UTCBen Kirchner

You say "he wasn't able to tell me" .. I assume you mean he was able to tell? If so, I suspect that there isn't just one "silver bullet," but rather a culmination of years (generations) of experience, seeing thousands of diamonds and becoming used to what they look like, and same with CZs, seeing thousands of them over the years.

I might be wrong though; there may be some obvious trick, or something not so obvious but a way to tell absolutely. Check out www.gemologyonline.com on the forums... Those guys have amazing knowledge about gems! Good luck :)

15th Sep 2009 06:33 UTCJenna Mast

Ben:

Yes, sorry, I did mean "was". :-)

15th Sep 2009 06:59 UTCDonald Vaughn

according to this site http://www.allsands.com/fashion/cubiczirconiad_xfv_gn.htm

the facet point in the wrong direction and the edges of the facets are not as clean also cz being synthetic

lacks inclusions.

to my untrained eye a CZ always seem's to be a brighter more colorful sparkle with more of the "Rainbow" effect than a diamond which seemed more white.

15th Sep 2009 08:39 UTCjacques jedwab

Caloric conductivity may be of some use to trained persons, endowed with a good Fingerspitzgefühl.

A pocket instrument based on that propriety is sold, e.g. by KRANTZ.

16th Sep 2009 17:59 UTCJim Ferraiolo

Main differences for the jeweler:

CZ has a higher dispersion that diamond, and the refractive index of CZ is lower than diamond.

16th Sep 2009 21:43 UTCRob Woodside 🌟 Manager

That's interesting Jim. Thanks. Dispersion usually increases with refractive index.

16th Sep 2009 23:21 UTCAdam Kelly

Diamonds also have sharper facet junctions whereas cz's have a more rounded facet junction.

If out of the mounting, cz's weigh about twice as much as a diamond the equivilant size.

Cz's don't have inclusions, and yes, years of looking at them helps to.

AK

5th Oct 2009 03:36 UTCJoseph Taggart, Jr.

Jenna,

I can't swear this is true, but a jeweler I met said that one of the cheapest ways he knew of for separating Diamonds and CZs was to put them in water. He said that the diamond "sparkles" even under water. I assume this has to do with Jim's comment about refractive index.

5th Oct 2009 17:04 UTCJan Styer-Gold

Yes, there is an obvious "trick" in telling the difference between a Diamond and a CZ. Due to the higher refractive index of a Diamond...you cannot read through it. You can read through a CZ! Place the stones in question over some written text, like a newspaper, table side down against the text. If you can make out the text...it's a CZ! Of course I would still check the stones with real testers too. Let's not forget about all of the Moissanite out there too. :)

Jan

5th Oct 2009 18:36 UTCRob Woodside 🌟 Manager

I'm a little puzzled by Jan's Comment. I could clearly see through my diamond octo into the matrix. If there was anything written there I could have read it.

5th Oct 2009 19:16 UTCJan Styer-Gold

Rob,

No puzzle... Sorry, I should have been more specific. This property mainly applies to faceted diamonds. Some irregularly shaped rough diamond crystals will also exhibit this property. I am able to see through and read through my octo crystals too...in or out of matrix. :)

Jan

5th Oct 2009 22:20 UTCRob Woodside 🌟 Manager

Thanks Jan

6th Oct 2009 00:17 UTCAnonymous User

According to my references:

CZ

Dispersion: 0.058-0.066. RI: 2.15-2-18. Density: 5.34-6.0.

Diamond

Dispersion: 0.044. RI: 2.417. Density: 3.515.

The thermal diamond probe is often used to compare thermal inertia, which is much higher in diamond than CZ or other simulants. This is done quickly without taking out of the setting.

The "read through" test is done with unmounted stones. They are placed table down on a printed sheet. The reason that this may be helpful is that a diamond is supposed to be cut to ideal proportions. This means that the pavillion facets are properly angled to allow them to reflect nearly all light entering the table back out the table. This angle, called the critical angle, varies with the refractive index of the stone. Therefore, a CZ cut to or close to ideal diamond proportions (which they usually are to look like they are cut correctly) would be cut with the wrong critical angle for CZ, and therefore not return as much light. Hence the read through. This same effect may be noticeable when viewed set from above as a dark zone - where light is not reflected from the pavillion and is allowed to leak out.

I'm not a jeweler. Perhaps one can correct or expand this?

6th Oct 2009 00:35 UTCRob Woodside 🌟 Manager

Thanks Ken what you say makes good sense

When Bob Downs was post docing in Washington, he got into the Blue Room at the Smithsonian where they have a fist sized diamond. Diamond has the largest heat conductivity of any material and it sucked the heat out of his hand when he held it. He said that's why they call it "ice" and not because of its appearance.

6th Oct 2009 01:04 UTCAnonymous User

Rob:

According to Arem (1987), diamond has a thermal conductivity of 1.6-4.8 cal/cm degrees C sec, while other gems vary from 0.215 (SiC) to 0.0012 (lead glass). Gold is 0.707, and silver is 1.00. Interestingly, Ag 69%/ Au 31% (weight) is 0.237. (Makes me wonder if you can judge karat by thermal conductivity.) I don't know if diamond has the highest conductivity, but it has to be up there.

6th Oct 2009 01:17 UTCRob Woodside 🌟 Manager

Doug Beder told me it was the highest 30 years ago. It could be one of those little time bombs of wrong things I carry around or old data. However your data do support it.

8th Oct 2009 01:48 UTCAdam Kelly

Rob, smaller diamonds are often place in the mouth, to check for the same thermal properties.

And as I was told, you are right, that's the reason it's called ice.

AK

8th Oct 2009 03:39 UTCJohn A. Jaszczak Expert

When I was in grad school I read about some synthetic diamonds being made with isotopically purified carbon.

The results were even higher thermal conductivity! (Less phonon scattering due to isotopic

impurities). A remarkable property since pure diamond is

an insulator. High thermal conductivity for metals (like copper) comes from the electrons. Diamond

has about 4 times better thermal conductivity at room temperature that copper. One reason is

that I think diamond is the most dense material in terms of atoms per volume. I think perhaps only

graphite has stronger bonds, but only in the graphene plane.

John

13th Nov 2009 04:34 UTCJohn Attard Expert

John,

The isotopically purified carbon you refer to is essentially pure carbon-12 in other words we are talking about carbon-13 depleted. Natural carbon is approx 99% C-12 and the rest mostly C-13 (and tiny amounts of the radioactive C-14). Yes the C-13 depleted diamond acts like you say and in addition it has markedly superior laser properties (quantum mechanical arguments) over regular diamond.

Back to thermal conductivity. Think about heat as atomic motion. Conductivity is due to atoms hitting neighbors and passing on motion to them on the whole going from high temperature to low. Not only does diamond have a lot of atoms per unit volume but in any diamond the atoms are all bonded together in a strong tetrahedral arrangement, a diamond is essentially a SINGLE molecule! The strong 3D structure accounts for extreme hardness. As for the electrical insulator properties? All its electrons are tied up unlike metals or graphite so no electrical conductivity.

16th Dec 2009 03:55 UTCAlfredo Petrov Manager

Just out of idle curiosity, what solid at room temperature would have the least number of atoms per unit volume? (a real solid, not some "air gel")

21st Apr 2010 10:14 UTCJamey Swisher

Just looking under magnification is something you pick up after years of experience. After about 5yrs I could ID most CZ at sight in a few seconds under magnification. There are too many things to explain that can be a telltale give away.

Also, not all CZ are cut to wrong angles. A CZ can still be cut to the "Ideal" cut and be in the correct angles for CZ. Ideal cut has nothing to do with angles, but rather proportions. Angles vary with every gemstones material, natural or synthetic. The catch is most CZs are either commercially cut or machine cut with lasers and not actually precision faceted, but that does not mean there are no precision faceted CZs, because many faceters cut designs in CA< I am one of them, lol, always test cut in CZ or other cheaper synthetic prior to possibly wasting expensive natural rough, and of course those CZ designs still get sold! But even commercially cut CZs have varying qualities! For example, Signity CZs are typically quite well cut and polished, unlike many other low end commercial cut CZs.

A few key things in faceted diamonds would be inclusions, naturals(areas or natural diamond crystal not faceted or polished on the stone, often near the girdle), etc. CZs will NOT have these.

With the progress into synthetic diamonds and HPHT treatments it is getting harder and harder to ID natural untreated diamonds!! In fact, so difficult, it is best NOT to accept/purchase a colored diamond anymore unless it comes with an origin of color report!!! Otherwise don't buy it as odds are it is treated to get the color, either coated or irradiated. There are also synthetics now that apparently can trick many diamond testers as well too!

Still one of the easiest way to ID a diamond from other materials, not synthetics though, is a simple specific gravity test, as long as the stone is loose of course.

But many gemologists and/or jewelers with lots of years experience can ID many a gemstone, diamond or colored, with simple magnification. Now, of course, you won't find a good one willing to do a valid ID from that alone, they will require other tests to validate what their eyes tell them, if not, it is one you want to run, not walk, away from and find another jeweler/gemo to take care of your needs, lol!!

It got to the point we invested in an Ocean Optics spectrophotometer now just to be able to have that extra power for ID'ing materials and/or treatments as well! Plus it really speeds up color grading as well, lol.

2nd Sep 2010 08:01 UTCcharles kraft

For an estate I had some jewelry checked, what the jeweler called a "granny box" check, superficial, but rather good I thought.

There were two rings, one CZ, one diamond; I was sure I remembered which was which. When he hit the CZ, he looked at if for about 2 or three seconds and decided what it was. When he hit the diamond he said "Someone was in the military in the early 1960s." He was right both times.

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