Diamonds at Auctions

Diamond expert Trine Lamm

A lot of people are asking me if it is worthwhile to buy diamonds at auctions. 

They have seen a super nice diamond, at a very low price at an auction house’s webpage. And they ask me “isn’t this bargain just too good to be true?”

And yes, it is probably too good to be true. There is ALWAYS a catch on a cheap diamond. 

So what do I look for when I look at diamonds at auctions?

First of all I look very thoroughly at the diamond, preferably in natural light. Is it pretty? Does it reflect the light evenly? Does it show sparkle and fire??? How is the condition of the diamond? Is it chipped on the sides?  

Normally diamonds on sale at an auction, are ‘graded-mounted’. This means that it is impossible to see all the inside of the diamond. It is impossible to grade a mounted diamond. You can have a look at it, and get a pretty good idea about the colour and clarity, but never the full version. 

When I grade a diamond, I always remove it from the mounting, clean it properly, and grade it, in a neutral environment, which means that I have the “diamond light” which essentially is a daylight-lamp. 

When I look at diamonds at auctions, I always look at the grading report the auction house has made, but it is very important to make your own judgement. 

Remember the 4 C’s of diamond grading 





In the information provided by the auction house, you should be able to see a colour grade, a carat weight, and a clarity grade. But remember that the diamond has been graded mounted. There can be inclusions hidden under the setting. 

The most important C is the Cut. The cut grade is never listed at diamonds on auctions and it is the most important. A diamond can have a beautiful colour, a good clarity and the perfect size. Yet a lot of the carat weight can be hidden in a bad cut. The diamond can have a very small diameter, but be very high. This means that the diamond will look smaller seen from above. Or it can be too flat, if the diameter is too big for the carat weight. A diamond cut too flat, will have what we call a “fish eye”, which is a darker ring around the centre, because it doesn’t reflect the light perfectly. 

For brilliant cut diamonds you should look for these diameters. 

A diamond of 1 carat should be 6.5 mm in diameter 

A diamond of 1.5 carat should be 7,5 mm

A diamond of 2 carats should be 8 mm

A diamond of 3 carats should be 9.4 mm

A diamond of 4 carats should be 10.4 mm

A diamond of 5 carats should be 11.2 mm

For fancy cut diamonds it is another story. When looking at a fancy cut diamond at an auction you need to judge if the cut is pretty. Is the oval cut diamond evenly oval, is it too long? Or is it too wide? Is the heart shape evenly? And pretty? Are the proportions correct in your opinion?  And so forth. 

It is very difficult to buy diamonds on auctions, you can be lucky, but get a professionals opinion before you bid on it. 

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