How Are Diamonds Graded?

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Diamond grading is a highly specialized skill requiring both human knowledge and experience, and technological assistance in the form of finely calibrated equipment. Diamond grading usually cannot be done using only human vision, even in the hands of an avowed expert. As an absolute minimum magnification to the power of 10 is required, and ideally grading should take place under laboratory conditions with access to, as mentioned above, specialist equipment.

But whether you are in a jeweler’s workshop or in a high quality lab, such as those operated by diamond grading authorities, AGS or GIA, there are strict and specific criteria that must be considered when grading the diamonds. They begin with four Cs which must be followed in the right order. Let us take a look.

The First Cut

A diamond’s cut is vital in showcasing the stone at its best. It will either show off the (nearly) perfect interior of the stone, or disguise its flaws; adding value (in the case of a round brilliant design), conserving as much stone as possible for a beautiful, budget friend result (as in a princess cut) or making the best of an awkward shape (for example, flat shallow stones fit nicely into an emerald cut). The cut is the first point considered when a stone is being graded because the cut is responsible for the brilliance and scintillation thrown out by the stone.

Colour Considerations

A ‘pure’ diamond should be completely colourless – invisible when placed in a glass of water, say – and even the slightest hint of colour can make a dramatic difference to the price of the stone. In general, diamonds are valued lower the more colour to be perceived in the stone – until you get right to the other end of the scale: Z and beyond, at which point the stone’s obvious colouration can begin to add value back in as the grading moves into the fancy and vivid fancy hues.

Here’s to Clarity

Clarity, when used in reference to diamonds, means the internal integrity of the stone. To put it another way, a diamond’s main value is due to the stone’s unique light reflecting and refracting qualities. Just as the cut is designed to show the stone off to its best, the clarity allows the stone to bounce light around inside it, allowing it to escape through the top of the stone, or the table – this is what makes diamonds seem to shine with their own inner light, sparkling and bouncing a scatter of rainbow light around them like an aura. If the clarity of the stone is obstructed – by a flaw or fracture in the stone – then the light display will be skewed or disrupted too. And, this, obviously, is undesirable in a diamond.

How Many Carats?

The final consideration is the size of the stone. Diamonds are slightly odd in that a small, nearly perfect stone can be worth a lot more than a flawed stone of five times its size, even though those flaws cannot be seen with the human eye, and sometimes not even unless the stone is in a beautifully lit laboratory, under stringent conditions. For this reason, the carat weight of the stone is almost an afterthought – when it comes to diamond grading, size really does not matter!


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