How did he do that? Diamonds vs CZ

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?



Edited 1 time(s). Last edit at 09/15/2009 05:32AM by Jenna Mast.

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

Ben Kirchner

Ben:

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

according to this site [www.allsands.com]
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.

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.

Main differences for the jeweler:

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

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

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

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.

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



Edited 1 time(s). Last edit at 10/05/2009 04:08PM by Jan Styer-Gold.

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.

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

Thanks Jan

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?

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.

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.

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.

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

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

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.