diamond grading

If you are considering a diamond with a carat weight of 0.3 carats or above, you should insist that the diamond has been properly certified. The certificate matters because it shows that the diamond has been graded at a laboratory and that its main characteristics have been tested and professionally assessed.

Not every grading lab assesses to the same standards, although rather confusingly, they may use the same terms. This means that if a certain lab company over grades a diamond, it will appear more valuable than it actually is. If the same diamond were then graded by one of the reputable grading labs, it is likely that the results would be much lower.

In the jewellery trade this problem is very well understood, and so reputable jewellery retailers will avoid the labs which have a reputation for deliberately over grading diamonds.

We recommend you choose a diamond graded by GIA because GIA maintain rigorous standards, they grade diamonds consistently, and it is a really good starting point from which to choose your perfect diamond.

At Samara James, we believe diamonds should be graded by reputable and independent grading laboratories like GIA. Find out why here.

The diamond is first cleaned using a special sapphire powder to make sure that not a single speck of dust could alter any of the clarity grades. It is then weighed in carats to five decimal places. This is the first of the four C's.

The diamond is then graded for colour. Each gemmologist has a master set of diamonds that are used to compare with the diamond they are grading. The colour grade is a measure of the hue of the colour tint of the diamond. The highest grade is D, which is colourless, and the lowest is Z. The diamond is positioned face down rather than face up so the gemmologist can evaluate the body colour of the stone.

The next stage is to assess the clarity of the diamond. This is done using a microscope. The clarity grade of the stone is given by looking at the size, position, and type of inclusions within the diamond visible from above. The gemmologist will then draw an inclusion map of the diamond with a top view and a bottom view. Internal inclusions are usually shown in red with external features usually drawn in green.

The last of the four C's is cut. The diamond is scanned into a computer using a laser, and each facet is measured. The computer takes an average of the angles and, if they fall within a certain region, they are given the appropriate grade of 'cut'.

Some consumers will want to know that not only is the average correct, but also that every single facet has been cut at the perfect angle. In this case, a round brilliant diamond can be tested for Hearts and Arrows. To test for Hearts and Arrows, the diamond is placed in a viewer, and a computer runs a series of tests to check that the diamond has perfect optical symmetry.

Once the diamond has been graded for its carat weight, colour, clarity, and cut quality, the certificate can be printed with its unique reference number. The diamond can then be sealed so it cannot be tampered with before leaving the laboratory and arriving with the retailer.

Description

If you buy an uncertified diamond, one of the first tests you may want to do is to check that the stone is real and natural and whether it has been artifically treated in any way. This video shows the process and equipment used by diamond grading laboratories to check each diamond.

Video Transcript

It is possible to test the diamond using an electronic diamond tester. To be 100% accurate though, you will need to send the stone to a laboratory where it can be tested in greater detail.

The grading laboratories use a piece of equipment developed by DeBeers which is called Diamond View. This instrument makes visible the growth structure of the diamond. The growth structure is very different between natural diamond and synthetic diamond or other types of stone. In nature, there is a continuous growth and dissolution of the diamond which gives an irregular growth pattern.

This is in contrast to what you see in a synthetic diamond which is growing in a systematic and continuous way. A diamond can also be treated to try to enhance its appearance. For example, a diamond can be heated, in order to improve its apparent colour, or it can be drilled with a laser and backfilled with glass to improve the apparent clarity grade.

To check if that has happened the laboratories use a technique called spectroscopy. This involves shining a light at the diamond at an atomic level and looking for the interaction and the light and diamond. A trained person can assess the reading to see if any ‘enhancements’ have been applied to the diamond.

Fluorescence becomes visible when you expose the diamond to UV light - it causes the diamond to glow, usually blue, but sometimes other colours too. Fluorescence is caused by gaps in the crystal structure of the diamond. The UV light excites electrons in the diamond and because of the gaps in the crystal structure these electrons can then vibrate causing the diamond to glow.

The diamonds are graded for the level of fluorescence from None, through to Faint, Medium, Strong and finally Very Strong. Higher levels of fluorescence can sometimes also cause the diamond to become milky or cloudy in sunlight. This reduces both the brightness of the diamond, and also the levels of fire in the diamond.

Unfortunately it is not as simple as saying that for example, any diamond with strong blue fluorescence will look milky or cloudy. It varies depending upon the diamond - if you were considering a diamond with higher levels of fluorescence it is definitely worth checking the diamond to ensure its appearance is not negatively affected by the fluorescence.

Colour is not a measure of the quality of a diamond, but rather it's rarity. In white diamonds, the better the colour grade the more rare the diamond. Diamond colour is caused by chemical impurities. For example, nitrogen will cause a yellow tint to the stone.

There have been diamond colour-grading systems since the early 1700s. But today, we use the standard that the GIA or Gemological Institute of America invented and developed in the 1950s. This scale starts with D and continues with increasing presence of colour to Z.

D colour diamonds are completely colourless and are the rarest and most chemically pure. Z colour diamonds are light yellow or brown and will usually sell for a much lower price. When a diamond is being graded for its colour, it is given to a gemologist with a master set of stones to compare with the diamond that they are grading.

The diamond is positioned face down, rather than face up so that the gemologist can evaluate the body colour without it being obscured by the brilliance of the diamond.

Some shapes of diamond, like the emerald cut, show up colour differences a little more due to the nature of the facet arrangement. This is evident in the face up position if the stones are examined carefully.

There is a degree of variance within each colour grade that can make a difference to the appearance of a diamond. The image on the left shows two G colour diamonds. They are both GIA certified as being G colour. However, the stone on the right is not quite as clear or white as the stone on the left.

As with all of the four C's, diamond colour is subjective. What may be right for one person may not be right for you. Some people find that lower grades, like J or K, are preferable as it adds a certain warmth to the stone, whilst others prefer higher colour grades for their purity.

Colour is important because it affects the transparency of the diamond and whether it has a yellow tint. So for example, if you choose a D colour, it will be colourless whilst a K colour diamond will have a hint of pale yellow. However, in a round brilliant diamond, provided you choose a colour that is H or above, it is the cut grade that makes a diamond look beautiful.

Compare these two diamonds - they are both perfectly cut diamonds with superb Hearts & Arrows. The only difference in specification is that one is a D colour and one is H colour. In this image, we are comparing the light return of the two diamonds. Both have excellent light return and excellent contrast pattern.

Comparing the levels of light refraction between the two diamonds, both have very attractive flashes of fire across the diamond. Both stones have similar levels of fire. In fact, it is very hard to pick the D colour from the H colour when assessing which is more beautiful.

To see the difference between these two diamonds, we need to turn them face down against a white background. The H colour has a faint hint of yellow whereas the D is completely transparent. Choosing which colour to go for then comes down to personal choice; if you want a better colour, it will be rarer and will therefore cost a little more.

In order to make sure that your diamond is as beautiful as possible, you should look for a perfectly cut Hearts and Arrows diamond. Provided you have chosen a stone from the near colourless range (H or higher), you will not notice any traces of yellow within your diamond.

When you look at a diamond certificate, you will often have a clarity characteristics line or an inclusion map with a key. This is telling you what the inclusions look like in your diamond and in the case of the inclusions map, where they are positioned within the stone.

We can explore the different types of inclusions, including crystals, needles, feathers, and naturals. The first type of inclusion to consider is called a crystal. Some crystals can resemble a diamond within a diamond, and some can resemble bubbles. Some crystals can be so thick that they start to take on a darker and even a black appearance.

A common inclusion type is called a cloud. This is a very broad term, as it means there is a collection of small inclusions close together. They can look, though, very different. If the diamond has a larger cloud, it may start to appear foggy or hazy, and this can have a significant effect on the appearance of the stone to the naked eye.

A feather is probably the easiest type of inclusion to identify and to find. A feather is a crack or a fracture within the stone that has the appearance of a feather. These are caused by immense pressure as a diamond has risen through the earth's crust. They are nearly always white, and they vary significantly in size and shape.

Needle inclusion: this is similar to the crystal inclusion, but as the name suggests, it is needle-like in shape. This type of inclusion is often white, and is very rarely visible to the naked eye.

An indented natural is where part of the skin of the rough diamond is left on the surface of the polished stone and that then intrudes into the diamond.

There are some other less common types of inclusions, such as twinning wisps. These are formed when a diamond stops growing and then starts growing again, maybe thousands of years later, but in a different direction. This change in grain leaves a mark in the stone, rather like the crown in your hair or a knot in a piece of wood.