How To Choose A Diamond
The Four Cs of Diamond Grading
We aim to make finding the perfect diamond an exciting and informative experience, sourcing individual stones to meet the different requirements and budgets of each of our clients.
Almost all of our diamonds of 0.3ct and above are supplied with laboratory certificates. These summarise how the diamond has been graded by an independent gemologist against four key criteria, known as the 'four Cs': carat, colour, clarity and cut. We've provided some guidance on the four Cs below, but we'd be delighted to talk you through them in person, and to show you a selection of loose diamonds of different shapes, sizes and grades. Please feel free to get in touch.
The size of a diamond is expressed in terms of its weight in carats, with one carat equal to 0.2 grams.
A diamond's value increases exponentially with size, meaning that a 1ct diamond will typically cost more than twice the price of a 0.5ct stone of similar quality.
Prices also tend to step up at the most popular weights such as 0.5ct, 0.7ct and 1ct, so you often achieve better value if you can find a diamond that you like just below one of these levels.
Confusingly, the same word ('carat' or 'karat') has a second completely different meaning within the jewellery industry, being used to describe the purity of gold. 24ct gold, for example, is pure gold, whereas 18ct gold is 75% pure.
Colour is graded alphabetically from D (completely colourless) down to Z (very strongly tinted). The visible difference between the premium grades is very subtle, but below H you may well start to notice an increasing degree of colour (usually a hint of yellow).
Beyond Z, diamonds are considered to be 'fancy coloured', which includes yellow, pink and blue diamonds. These are graded from Light through Fancy Light, Fancy, Fancy Intense, and Fancy Deep to Fancy Vivid.
If you're looking for a white diamond that will not show any obvious colour in a platinum or white gold setting, we recommend focusing on stones graded G or better. Of course, you may choose to restrict your search to higher colour grades, but assuming your budget is not unlimited, the higher the colour grade the more you may have to compromise elsewhere.
Almost all diamonds contain natural imperfections. These can include blemishes on their surface as well as inclusions within their crystal structure. Gemologists label different types of imperfections with names such as 'clouds', 'feathers' and 'needles', but they all result from small crystals becoming trapped inside diamonds as they form, and diamonds developing irregularities in their atomic structure as they grow.
The clarity grade of a diamond is an assessment of how significant these imperfections are when viewed at 10x magnification.
Clarity grades range from IF (internally flawless), meaning that no internal characteristics are visible at 10x magnification, down through Very Very Slightly Included (VVS1 and VVS2), Very Slightly Included (VS1 and VS2), Slightly Included (SI1 and SI2) and Included (I1, 12 and I3).
A diamond graded VS2 or better should not have any inclusions that are visible to the naked eye. Stones graded SI1 or below may not be 'eye clean' – sometimes they may look clean initially, only for inclusions to become apparent later, once the diamond is no longer spotlessly clean and sparkling quite so brightly. Of course you can always clean the diamond again to restore its lustre, but it is difficult to 'unsee' an inclusion once you know it's there.
Confusingly, the term 'cut' has several different meanings in the context of diamonds.
Firstly, it is used to describe the shape of a diamond — round cut vs oval cut for example. Diamonds can be cut into a wide variety of shapes. By far the most popular is the round brilliant cut. All other shapes are collectively known as 'fancy cuts', the most popular being:
Secondly, the term 'cut' is used to describe different styles of diamond faceting: 'step cut' vs. 'brilliant cut' vs. 'rose cut' for example.
Square or rectangular stones whose facets are cut in parallel straight lines are known as step cuts, the most famous being the emerald cut. Step cuts prioritise weight preservation and the reflection of light over refraction. In practice, this means that they are more inclined to exhibit flashes of white light than to sparkle.
Brilliant and modified brilliant cuts on the other hand have triangular and kite-shaped facets which increase the refraction of light, creating more of a sparkle, but at the expense of weight loss and a reduction in the amount of reflected light.
Mixed cuts share aspects of both brilliant and step cuts, combining some of the weight preservation and dimensions of step cuts with the sparkle of brilliants. The most popular example of this is the princess cut.
Rose cuts are very different from both step cuts and brilliants, having a flat base, and are very rarely seen these days, except in antique jewellery.
Finally, in the context of diamond grading, the 'cut grade' is an assessment of the proportions of a round brilliant cut diamond.
The proportions (the diameter vs the depth for example) determine the angles between the facets of a diamond, which in turn determine how well a diamond reflects and refracts light – and therefore how much it flashes and sparkles.
There is no consensus on the optimal proportions for fancy cuts and so reputable gemological laboratories do not ascribe them a cut grade as such. They are however given 'polish' and 'symmetry' grades (as are round brilliant cuts).
Different gemological laboratories use slightly different terminologies, but as a rule we advise clients to focus on stones with the top one or two grades for cut, polish and symmetry. The price premium is usually relatively modest, and the impact it has on the appearance of a diamond is usually well worth it.
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