Posted on: April 6, 2020

Beginner’s Guide: buying diamonds at auction

In this week’s beginner’s guide, we’ll be deciphering diamonds and the essential features you should consider before buying a piece of diamond jewellery at auction.

Art Deco diamond brooch sold for £2600

Since their discovery in India in the 4th century BC, diamonds have captivated us for their beauty, durability and mystery. In Indian culture they were used as talismans, believed to ward off evil and protect their wearer in battle. But it wasn’t long until diamonds gained notoriety across the world for their extraordinary physical and optical properties. 

One of diamond’s most remarkable qualities is it’s hardness. The word ‘diamond’ is derivative of the Greek adjective ‘adamas’ meaning unconquerable or indestructible; to this day, no natural material has been found that supasses the hardness of diamond. Raw diamond crystals became a useful working tool for cutting and engraving, and they are still used in industrial equipment today. Their hardness and durability also make them a fantastic gemstone, as they can be worn daily and obtain minimal damage long-term. Of course, diamonds are also valued for their optical properties i.e. the remarkable way they interact with light; as well as being exceptionally bright and reflective, diamonds are noted for their ability to split light into its spectral colours, producing a dazzling rainbow effect known as ‘fire.’

For these qualities, as well as their relative rarity, diamond became and still remains one of the most highly valued gemstones on the market today. But not all diamonds are valued equally, and many factors are taken into consideration when assessing the value of a diamond. Whether you’re looking to buy for yourself or a loved one, or simply want to know a little more about this alluring gemstone, here’s a few tips on what to look out for at auction:


To the naked eye, the vast majority of diamonds (approx. 98%) appear colourless. However on closer inspection, most diamonds show a mild to moderate draw of yellow. This colour is caused by nitrogen impurities, which are very common in diamond crystals. The higher the concentration of nitrogen in a diamond crystal, the more apparent the tint of yellow in the stone will be. In the diamond market ‘whiter’ diamonds are favoured over those with a yellow tint, however stones which are free of, or do not possess significant numbers of nitrogen impurities to impact colour, are rarer and hence more valuable. 

‘Colourless’ diamonds are graded on a scale of D to Z, with D meaning truly ‘colourless’ and Z meaning ‘light yellow.’ Generally, a diamond with a colour grade of around K or less will have a yellow tint which is obvious to the naked eye. Two stones of the same size, cut and clarity, but with notably different colour grades, can have a difference in value of hundreds or even thousands of pounds. 

Diamond 2.80ct solitaire ring sold £4300

Colour grading a diamond can only be done under specific lighting conditions by a qualified diamond grader. Even then, this is an assessment based on the stone as it appears in it’s setting. An ‘official’ colour grade can only be determined when the stone is removed from its setting and assessed at a gemmological laboratory, which will issue a laboratory report stating the stones official colour grade. When shopping for a diamond at auction, be sure to ask if the diamond has a laboratory report stating it’s official colour grade. If not, ask if a qualified representative of the auction house can provide an estimated colour grade for you.

Occasionally, you may come across much rarer ‘fancy’ coloured diamonds – these are stones which show unusual hues of colour, including strong hues of yellow, blue, pink, green and orange. Unlike their ‘colourless’ counterparts, coloured diamonds are exceptionally rare and are graded on an entirely separate scale. Generally, they are valued more highly if they show a stronger draw of colour. A pink diamond can be worth significantly more than a top grade ‘colourless’ counterpart of the same carat and clarity. If you fancy something a little less traditional, be sure to check that the diamond has been assessed as natural and untreated – due to their high value, coloured diamonds are often artificially reproduced. 


To the naked eye, the majority of diamonds appear to be transparent. However, as with colour, this is often not the case on closer inspection. Under magnification, blemishes and imperfections known as ‘inclusions’ can often be found inside the diamond. Inclusions can come in many forms – from opaque black ‘carbon spots’ to translucent white cracks or ‘feathers.’ Inclusion free diamonds are more favoured on the diamond market, but again are relatively rare, making less included diamonds more valuable. 

A diamond’s clarity is graded according to the number of inclusions within the stone, their size and their location. These are graded on a scale from ‘FL’ meaning ‘flawless’ (which require laboratory assessment to assure their are no internal impurities) to ‘I’ meaning included – often inclusions in ‘I’ graded stones are so noticeable, they can be seen with the naked eye or risk the integrity of the stone. The difference in value between a ‘flawless’ stone and an ‘included’ stone can be hundreds or thousands. 

Like colour grading, a diamond’s clarity can only be assessed under specific lighting conditions by a qualified diamond grader. Even then, this is an assessment based on the stone as it appears in it’s setting. An official ‘grading’ can only be provided when the stone is removed from its setting and assessed by a gemmological laboratory, which will issue a laboratory report stating the stones official clarity grade. When shopping for a diamond at auction, be sure to ask if the diamond has a laboratory report accompanying it stating the official clarity grade. If not, ask if a qualified representative of the auction house can provide an estimated diamond clarity grade for you.


Art Deco diamond pendant sold for £1400

Diamond cuts have evolved greatly since their initial discovery in the 4th century BC. When they were first discovered, the raw crystals were set in jewellery as found, as there was no material strong enough to cut a diamond. Later it was discovered that diamond crystals could themselves be utilized to cut other diamonds, and from this point the art of diamond cutting began to evolve. 

Over the centuries, diamond cutters experimented with styles that would show off the unique qualities of a diamond to best effect. In 1919, Belgian cutter Marcel Tolkowsky created the round-brilliant cut, comprising 57 facets at specific angles which showed off the brilliance and fire of a diamond in perfect balance. Over 100 years later, the brilliant cut remains the most popular diamond cut used in modern jewellery.

Assessing the quality of cut in a diamond is often overlooked, but can make all the difference to a stone’s appearance; a diamond with poor symmetry and proportions can look dull and lifeless, despite being a good size, colour and clarity. A good cut will give a diamond the sparkle and fire that makes a diamond so appealing.

When shopping for diamonds of a more unusual cut, such as emerald cut, pear-shaped or oval, stones won’t always display the same brilliance and fire as they might in a brilliant cut, but aspects such as symmetry and proportion should still be considered. Even with smaller diamonds, a well cut stone is a mark of excellent craftsmanship and can affect the overall look and value of a piece of jewellery greatly. 


Carat refers to the weight of a gemstone. The word ‘carat’ derives from the word ‘carob.’ In the middle east, carob seeds were used as counterweights to weigh precious metals and gemstones. A one ‘carat’ diamond is equivalent in weight to one ‘carob’ seed, which is approximately 0.2 grams.

Typically, stones of a greater carat weight (and therefore size) will have a greater value than that of a smaller stone. But this should always be considered in conjunction with factors such as colour and clarity; a stone weighing 1ct of optimum colour and clarity can be worth much more than a 2ct stone of poor colour and clarity. 


A pair of diamond drop earrings sold £8000

Once you’ve established the quality of the diamond itself, turn your attention to the stone’s setting. While a piece of jewellery may have the perfect stone, it’s setting could be a major drawback in it’s appeal to a buyer. It is not uncommon for bidders to buy a jewellery piece for the stone itself with the intention of resetting it. However, this can be expensive, and buyers should be wary to get a quote from a jeweller before making this decision to avoid any shocking additional costs.

One of the most common issues is the type of metal in which the stone is set. In today’s market, we’re seeing a preference for white metals such as white gold and platinum over yellow metals. Depending on the piece, it may be relatively simple for a jeweller to ‘convert’ yellow metal to white metal – rhodium platings can be added to give a white metal finish, but the cost of this should be considered before buying. It’s unlikely when buying antique and second hand pieces that you will find a piece that is exactly the right size, so re-sizing may be necessary. However, be mindful of the shank or chain of the piece if buying a ring, bracelet or necklace – resizing is easier to do on some pieces then others. On a ring for example, shanks set with diamonds or other gemstones, such as the popular ‘eternity’ style rings, are much more difficult to resize and could cost far more to adjust.

If you’ve found a piece that’s just right for you, make sure you check how secure the settings are – on antique pieces particularly, wear to the prongs securing the stones is relatively common, and the risk of the diamond becoming loose is greater. Worn settings can also expose the girdle of a diamond, which is most vulnerable to chipping and damage. This can be relatively simple for a jeweller to fix, but is an additional cost to consider.


While buyers should be wary of the technical aspects of diamond valuing before making a purchase, jewellery is a deeply personal and sentimental commodity, and investment value isn’t always the top priority of the buyer. Some bidders may prefer a smaller stone of better colour and clarity that may retain more value long-term. Others may look at a piece of jewellery purely for its aesthetic value and may prefer a slightly lesser quality stone if it is larger and well cut diamond with a bit more sparkle. As with any area of antique collecting, the best advice we can give you is to buy what you love – in that sense, you can never be wrong!

To keep up to date with the latest news and posts from Trevanion & Dean follow us on twitter and facebook. For inquiries please email