One of the most common questions that gemmologists are asked is how to tell the difference between a real diamond and a fake stone.

Debra Goddard paid £10 for a ring at a car boot sale 33 years ago but only recently discovered its astonishing value. The 55-year-old mother had bought the expensive piece of jewellery for a bargain price at the age of 22, but it’s astonishing worth only came to light in February, when Sothebys valued the 25.27 carat gem at £740,000. 

Debra had taken the ring to the jeweller’s hoping that it might be worth a couple of hundred pounds. Little could she have imagined the true value of the ring. 

Bearing in mind Debra’s remarkable story, the Islander spoke with diamond expert Mark Johnson from Serendipity Diamonds, who works with diamonds on a daily basis at their Ryde showroom. He revealed the secrets of his trade and told us how he can tell whether a diamond is real or fake. 

Mark explains: “A great deal of estate jewellery comes up in our line of business. People who are selling their parents’ estate sometimes assume their worth is just the house. However, it’s quite common to find items inside the home which are a lot more valuable than was originally thought.”

Mark gave us the following advice on how to distinguish between real and fake diamonds. Here are a few simple DIY tips Islander readers can follow to check out gems they inherit or find at car boot sales. It’s well worth taking in the following points before paying for the expense of a professional gemmologist to value the jewellery. 

So, how can we tell how much the ring Aunt Agatha left in her will might be worth and whether it’s genuine or costume jewellery? Here are just a few techniques to help you know whether the object you have in mind is a true diamond or not.

1. Look at the diamond and setting through a 10x loupe

A loupe is a magnifying glass that allows you to look closely at the gem and its setting.

Mark says: “When looking at a diamond, you’ll notice a few things. Firstly, look closely at the diamond’s edges. When observing a diamond through a loupe, a real diamond has sharp edges. By comparison, a Cubic Zirconia has more rounded edges. 

“Secondly, most diamonds are natural, which means there may be some flaws. Small characteristics within the diamond, called inclusions indicate the stone is a real diamond. A fake stone often looks absolutely flawless and incredibly white. Only a very small proportion of natural diamonds achieve a Flawless grade.  

“Finally, examine the metal and hallmark of the mounting. Pay particular attention to the hallmark that might signify which metal was used. If the precious metal is Gold, or Platinum, there is more chance that it might be a diamond. 

2. Is the stone badly worn?

When viewed through a loupe, most diamonds appear the same as the day they were cut, unless chipped or broken. Other diamond lookalikes have far less durability and reveal scuffs, abrasions and wear across the stone. This may identify the stone as a fake. 

3. Try the ‘fog test’

The ‘fog test’ involves breathing hot air on the diamond as if you were fogging up a bathroom mirror.

Mark says: “A fake diamond will fog up for a short period of time. A real diamond will clear instantly owing to diamond being a good thermal conductor. 

4. Is the stone refractive?

Glass, quartz, and other imitations may mimic the brilliance of a diamond, but are far less refractive. When a genuine diamond is placed over a newspaper, the light will scatter inside the real diamond and prevent a black reflection. A fake diamond will let the black shine right through. You may even be able to read the article through the stone. Also, if your diamond is mounted, it’s a very bad sign if you can see through the diamond to the mount itself.

5. Weight. If your stone is loose and not set, weighing the stone will reveal if it is a diamond or not. Many diamond ‘lookalikes’ such as Cubic Zirconia weigh heavier than a diamond of the same size and shape. 

Having completed the home tests, if you need to be absolutely certain about whether your diamond is genuine or a fake, then you should take it to an experienced jeweller. 

Mark says: “Every week we have visitors dropping into our showroom to identify if diamonds are real or not. 

“One client was offered £500 for a ‘diamond’ they found whilst on holiday in the Far East. They refused the offer, suspecting the gem was worth far more. When they brought it to us, we identified it as a fake. 

“Don’t just take your diamonds to the nearest jeweller. Do your research and find a diamond specialist. 

“At many jewellers you’ll find sales people rather than gemmologists. They know what sells in their shop and what customers generally like. Go to a jeweller’s you can trust and ask for a reputable gemmologist who knows about diamonds.

“Even if you know the jewellery contains a genuine diamond, it’s still worth trying to find their true value.Take a 1-carat diamond, for example. The cut, colour, clarity and carat weight (also known as the 4 C’s) will determine whether it is worth £800 or more than £20,000.

Imitation diamonds

Cubic zirconia has been mass-produced since 1976. Unlike genuine diamonds, cubic zirconia scratches easily.

Moissanite is harder than cubic zirconium and visually dazzling. Unlike true diamonds, you can see the colours of the rainbow within the stone.


White sapphire is usually blue, but this gem can also be white. Sapphires are more prone to more damage than diamonds and do not have the same fire and brilliance.

White topaz is a mineral usually tinted yellow, red, brown, or pale grey, but can sometimes be white or appear colourless. Topaz is not as hard as diamonds. The gem is liable to wear down and scratch over time, giving it a dull or cloudy appearance.

Lab-grown diamonds are technically ‘real’ diamonds both chemically and physically, but far less expensive than those that have been mined. Mark claims their value is  20% to 30% less than that of a traditional diamond.

Now, having learnt how to distinguish between fakes and the real thing, the next time you come across something you think is cheap costume jewellery, please test it. It may be worth far more than you could ever possibly imagine.

Colin Clarke
Author: Colin Clarke