How to tell the difference between a Diamond and a Cubic Zirconia (CZ)

How to tell a Diamond from a simulant is one of the questions I get asked a lot and as I teach my students how to do this, I thought I would create a short how to guide on how you can tell the difference between a Diamond and a Cubic Zirconia, you may need to use a few of the tests in this post to be sure you get a definitive answer but when it comes to gemstones, it is always better to be safe than sorry. You will also need to check to make sure that the stone is not a synthetic moissanite and I will cover that in another post.

Equipment you will need:

Lined Paper

A jewellery cleaning cloth

A Jewellers Loupe with 10x Magnification

A pair of diamond tweezers

A thermal diamond tester (optional)

A diamond and moissanite tester (optional)

Before we start….

Always make sure the stone is clean before doing any of these tests as dirt on the surface of the stone can give you incorrect results. If the stone is loose, a simple wipe over with a cleaning cloth will work, if the stone is set then putting the piece in an ultrasonic cleaner will give it a good clean, if you do not have an ultrasonic cleaner, a soft toothbrush and warm soapy water will do the job.

Test Number 1: The Read Through Test

Now I am sure you are wondering why I have included a piece of lined paper as a piece of equipment, well there is one really easy test you can do but it does come with two limitations 1) this can only be done with loose stones and 2) it can only be done with round brilliant cut stones. So if you have a loose round brilliant cut stone, you can do what is called the ‘read through’ test and all you do is place the stone on the line of the piece of paper and see if you can see it through the stone.

Read through test for diamond and cubic zirconia

As you can see in the image above, you cannot see the line through the stone on the left, this is because round brilliant diamonds are cut in a way that they have what is called total internal reflection and this causes any light or visuals entering through the table of the stone to be reflected within the stone and they go back out through the table. If the diamond is not cut to ideal proportions, you may be able to see some of the line through the stone but most of it will be reflected back out of the stone. As you can see with the stone on the right, you can still quite clearly see the line through the stone due to the fact it has been cut to look like a diamond, rather than to produce total internal reflection.

Test Number 2: Look at the finish of the girdle

As with test number 1, this test is only really useful when it comes to round stones but it is a really useful and easy test to do. First off, what is the girdle? The girdle is the perimeter of the stone where the crown (top) and pavillion (bottom) of the stone meet. When trying to identify if the stone is a diamond or cubic zirconia, examining the finish of the girdle can help you, you may be able to do this with your naked eye or you may need to use your 10x loupe.

The two images above are what you may see when looking at the girdle condition of a polished stone. The image on the left is what is called bruted and this is only seen on diamonds and it has a frosted and satin appearance to it, where as the image on the right is what is called grounding and this is where they grind the girdle of the stone and this can be seen by many small vertical lines running along the girdle, this is only done to cubic zirconias. Unfortunately they can finish girdles in other ways, such as polishing or faceting them, if this is the case, the other tests will need to be done. What I like about this test is that it is quick and easy to do and can be done if the stone is in a claw setting. With any stones that have straight edges such as princess, emerald or baguette cut, the girdles will always be polished or faceted.

Test Number 3: Look at the surface condition of the stone

This is the thing I teach people to do the most when trying to decide if the stone is a diamond or cz as it something that it easy to do and only requires basic equipment (loupe and tweezers if the stone is loose). Diamonds are the hardest natural material known to man and sit at the very top of the MOHS hardness scale at 10, as they are so hard, it takes time and effort to polish them so that they have that lustre that everyone expects when they see a diamond. This means that the finish is usually done to a very high standard and all the facet edges are sharp and precise, all the points meet up with each other and there are no easily visible polishing marks on any of the facets and this can be seen in the image on the left below. Where as the image on the left, the facets don’t look as precise, sharp or well finished, this is because CZ’s are cheap and it is not cost effective for the stonecutters to make sure every stone is perfect, as long as it looks good then it will do. Often on CZ’s, you will see misaligned facets and often extra facet edges to compensate for the misalignment. There is also a higher chance of polishing marks being visible on the facet faces.

You may also see more damage on a CZ than you would a diamond as they are softer, only scoring 8.5 on the MOHS scale. This means that they are more likely to get chipped or abraded along the facet edges and faces than a diamond. Now I am not saying that diamonds can’t be chipped or abraded, but there is far more chance of it happening with a CZ than a diamond. The facet edges may also be a little more rounded than a diamond as being softer means they don’t take such a good final polish. So of you see that the stone is poorly cut and has damage at a few points around the stone, this is a good indication that the stone is a CZ.

I would like to note here that Swarovski now do a range of Cubic Zirconia that are cut to a really good standard but they have been very kind and laser inscribed Swarovski across the table to help us identify them.

Test Number 4: Look for inclusions

This is something that gets overlooked by many jewellery professionals but it really is a simple thing to do. Using you jewellers loupe and tongs if the stone is loose, have a look through the table of the stone (the large facet on the top of the stone) and see if you can see any inclusions in the stone, I have included a link below to the GIA’s list of inclusions as there are quite a few:

GIA list of inclusions

Now you may not see any inclusions as it does take some practise and experience to be able to easily spot them, especially in higher clarity diamonds but if you do quickly and easily spot an inclusion in the stone, that is similar to those on the GIA list, then there is a good chance that you have a diamond as the majority of cubic zirconias have really good clarity due to the way they are made.

Test Number 5: Using a diamond tester

I can nearly hear people already saying why have you left this till the end!! This should be the first thing you should do!!

But and this is a big but, being able to identify a Cubic Zirconia without using a diamond tester is a really useful skill to have as there are going to be times when you haven’t got your tester or it has broke and you have no clue what you are looking at and I was also taught that using your probe was the last thing you should do and that is the way I now teach. Also, diamond testers aren’t that cheap and if you only have a couple of stones to test, it may not be cost effective to buy one.

What is a diamond tester? A diamond tester checks the thermal conductivity of the stone as diamonds are thermally conductive where as CZ’s are not. However these do not confirm that the stone is a diamond as synthetic moissanite is also thermally conductive. That is why many jewellery businesses are investing in the dual probes to help their staff.

How to use a probe? I have seen many people using these probes incorrectly, which has resulted in the getting the wrong results.

  1. First things first, switch the probe on but wait until the light stops flashing before you use it, trying to use it while the light is flashing won’t give you any results.
  2. Secondly, test that the probe is working, touch it on a piece of metal and make sure it gives the metal response. If this gives no response then there may be a problem with your tester.
  3. Finally, you want to place the probe on to the stone correctly at this is at a 90° angle to the table of the stone, if the stone is loose then make sure you place it in the metal stone holding tray provided *DO NOT hold the stone between your fingers when testing it as the heat from your fingers can give you an incorrect result*. Press the probe down so that they probe retracts slightly into the tester. If you get no response, then you have a CZ or another simulant, if you get a positive result you have a diamond or synthetic moissanite. Sometimes you have a half result, where half the lights light up and this result is most often returned by Sapphires as they do conduct some heat but not as much as a diamond or synthetic moissanite.
  4. If you use a dual tester, the tester will tell you whether you have a diamond or a synthetic moissanite. If you get no result from the tester, then you will more than likely have a CZ.

Conclusion

I hope the tests mentioned above are helpful and you can use them to help identify what stone you have. If after doing all these tests, you are pretty sure that you have what could be a diamond. You will need to do a few more tests so that you can differentiate whether the stone is a diamond or a synthetic moissanite.

It does take some experience and time looking at different stones to be able to accurately identify what stone you have and reading a guide like this (even though it is pretty good!) is still not enough without practical hands on experience. If you are still not sure what kind of stone you have, then it is best to take the stone to an experienced jewellery professional who will be able to give you a definitive answer.