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When selecting diamond pairs it's important to select two diamonds which appear and perform similarly. However this does not mean that a pair has to have every characteristics matching to the another. There can be unlimited number of combinations that a diamond has and can be paired with another. This means that you don't have always target same carat weight, clarity or even colour (but care needs while selecting colour, more on this later). Note that I haven't included cut. That's because while Excellent cut range of GIA is still slightly broad it's important that you always stick to the best cut possible as that ensures your diamond is sparkly and brilliant.
You can select a diamond pair with each one having a slightly different carat weight as it's more important for two diamonds to appear in size and performance similar to the other. So a pair can have diamond weighing a 53 points and a 51 point and both can appear and perform similar. What influences the appearance and performance is the cut (even within the Excellent range).
With clarity there's some room and a diamond pair can have two diamonds with different clarities but I prefer to keep diamonds within one clarity up or down. This ensures that the value of two diamonds stay within the same price range. That means a VS1 can be pair paired with one below at VS2 or one up at VVS2, or a VS2 can be paired with one up with a VS1 or one down with an SI1. Also when selecting diamonds in lower clarity ensure that the diamonds are free from eye-visible inclusion.
Colour - Like with Clarity - You can go one up or down with colour if you are selecting pairs for an earrings because these diamonds are going to worn at a fair distance and won't come into comparison as with a diamond pair is set side-by-side. However again for the same reasons as above you should stay within one colour difference and should be careful when selecting diamond pairs in lower colours as there can be a fair bit of colour difference within one colour in lower coloured diamonds as the range tend to increase when the colour range goes towards J colour and below. Also with lower coloured diamond the tint of the diamond should be ensured - a K with Faint Brown comment can't be paired with a K with no comments. (Info: Grey tint diamonds don't receive a colour grade below J, but instead only a Faint Gray, Light Gray and so on, grade).
Appearance & Performance - This is by far one of the most important factor in selecting diamond pairs. Two diamonds in a pair should appear and should have similar performances. To achieve this a whole lot of factors come into play including the table size which is perhaps one of the most important of the elements that influences how a diamond appears (defining the flavour of the diamond). In short a diamond with Ideal proportions should be paired with another diamond having similar proportions. This means a diamond with Ideal proportions will not make a good pair with a diamond that has say 60/60 proportions. Another very important factor is face up size of the diamond - diamonds in a pair should have similar diameters. Notice I use the word similar - so you don't have to find two diamond that have exact diameters but a margin of 3-4% is acceptable when selecting pairs - as that kind of difference would not be legible to the eye and much less when the diamonds are being worn at two difference places (one in either ear).
Hopefully this guide gives you a starting point at selecting diamond pairs. At Swanstar we would be happy to select a diamond pair for you should you find the above a little too much to work on!
If you have any comments or suggestions relating to this topic feel free to write us.