How to tell if a diamond is real or fake


Prior to the late 1970s, if you were lucky enough to be given a diamond you could be categorically certain that it was genuine. The reason for this was that up until then there was no diamond substitute. However, in 1977 all that changed with the introduction of cubic zirconia in the jewellery marketplace (and, more recently moissanite, which was introduced in 1998). Due to its flawlessness and relatively low price, cubic zirconia has become an incredibly popular jewellery option for people all across the world. So, what can you do to check whether or not a diamond is indeed the real thing or a very high quality and convincing fake? Is it even possible to tell the difference just by eye?


Firstly, if the diamond was bought brand new from a reputable jeweller then it should really have been accompanied by a certificate from the Gemological Institute of America (GIA), which would confirm exactly what type of gemstone it is. If it did not come with a certificate, yet was bought brand new from a jeweller, then it would be wise to contact them and enquire about GIA certification.

If the diamond was bought second hand (or given as a gift) and no longer had its certificate and you wanted to check if the stone was real, then we would highly recommend that you take the gem to a jewellery industry professional to have it appraised, and to order a replacement certificate. However, if you are unsure as to whether or not to get the gem professionally appraised there are a few simple tests which can be conducted by yourself which might help you to make up your mind.

Use Your Eyes

Diamond has a higher refractive index than cubic zirconia (the refractive index is a measurement of how difficult it is for light to travel through something – the higher the refractive index number the harder it is for light to travel through). Diamond has a rating of 2.42 on the index, whereas cubic zirconia sits between 2.15 and 2.18. Looking closely at cubic zirconia, you would most likely see light giving off a prismatic effect (known as ‘fire’), whereas with diamond light bends sharply; so sharply, in fact, that if you were to try and look through a diamond you would not be able to see the stone’s bottom point. If your gemstone is loose, i.e. it is not mounted, then a popular method of testing is to place the stone, head end down, upon a sheet of newspaper. If you can see the print through the gem, then that is an indicator that the stone could be a fake.

As mentioned above, one of the reasons why cubic zirconia has become so popular is due to its flawlessness – but therein, as if by some ironic twist of fate, lies its flaw. Finding a truly flawless diamond is exceptionally rare. Almost all diamonds, including those considered to be the most beautiful and valuable, will have some impurities within their lattice structure due to the mining process. These impurities (known as inclusions) are normally in the form of a pale brown or yellow tinge, which will contain tiny flecks from other minerals. Because cubic zirconia is man-made, within sterile laboratory environments, it will not have any type of inclusion.

Use Your Hands

Cubic Zirconia is a far denser material than diamond. Cubic Zirconia has a specific gravity of 5.6 to 6.0, whereas diamond has a specific gravity of 3.52, making a cubic zirconia stone around 1.6 heavier than a diamond of the same size. If you have two un-mounted stones and know for certain that one is either a real diamond or cubic zirconia, then bouncing both stones gently in the palm of your hand should give you a pretty clear indication if the stone is indeed a diamond or cubic zirconia.

Use Your Mouth

It is odd to think that you could potentially tell whether a diamond is real or not by using your mouth, but it is possible. Diamond is a thermal conductor, whereas cubic zirconia is a thermal insulator. This means that if you were to breathe on the stone (in much the same way as you would if you were about to clean a pair of dirty glasses), then if it really is a diamond it will not fog up, whereas cubic zirconia will.

These simple tests are useful to know if you have any suspicions about a diamond, but they will not conclusively confirm whether or not the stone is real. For this it is always recommended that the stone be professionally appraised by a jeweller. Jewellers will employ a variety of methods during their appraisal, including running an electrical current through the stone by using a Diamond/Moissanite Detector, measuring the thermal conductivity of the stone with a Thermal Conductivity Diamond Tester (after heat is applied a diamond will be almost instantly cool to the touch, whereas cubic zirconia will retain some external heat), and of course checking the stone against the industry recognised grading standard, The 4 C’s: colour, cut, clarity and carat weight.