Diamond buying guide

Buying diamonds is not something to be rushed. Not only is it likely to be a significant financial investment, but making the right choice is important as your diamond will be treasured and admired by you and your family for generations. At Harper Tait, we believe in helping our clients to make an educated, informed purchase. A qualified gemmologist will explain to you the different factors that affect the value of your diamond in clear and understandable terms. We do not employ sales staff to pressurise you into making a quick decision, but rather expert advisers with many years of experience who want to ensure that your eventual choice is right for you.

The GIA grading system

At Harper Tait, we use the diamond grading system formulated by the Gemological Institute of America (GIA), the most respected and widely-recognised diamond laboratory in the world. The GIA grading system was developed in the 1950s, though in many respects it uses the same principles as the first ever diamond grading system as used in India over 2000 years ago.

Today the GIA system is effectively a universal language used by all diamond professionals and is the most rigorous and accurate measure of grading diamonds according to the four C’s- colour, clarity, cut and carat weight. Each diamond we supply will have a grading report that forms the basis of its objective value on the international market.

Every diamond is unique

Diamonds come in many sizes, shapes and colours, and all have distinct variations that make them as different from each other as individual snowflakes. All polished diamonds are valuable, but some are worth more than others. To a large extent the value of a diamond is based on its rarity, and this, in turn, is determined by the presence in the diamond of several different factors.


The skill with which a diamond is finished can greatly affect its value and quality. A well-cut diamond will dazzle with its beauty due to the way it interacts with natural light, giving off a brightness and colour that is extremely pleasing to the eye. This interaction is determined by the proportions and polish of a diamond, as these features maximise the potential optical effects created by a well-cut diamond.

These effects are known as brightness (reflections of white light), fire (flames of colour) and scintillation (areas of light and dark). The relative size, arrangement and contrast of these light and dark areas, caused by internal and external reflection, make up a diamond’s pattern. Ideally, this pattern should give a diamond a sharp, crisp appearance.

There are five grades of cut – excellent, very good, good, fair and poor. The higher the cut grade, the brighter the diamond. Harper Tait only uses cut diamonds that are graded excellent or very good.

Carat weight

A metric carat is equal to 0.2 grams or 0.007 of an ounce. One carat is divided into 100 points. Diamonds are weighed to one-thousandth of a carat and then rounded up to the nearest point, expressed as carats and decimals (e.g. 1.05 carats) or points (e.g. 75 points). A fraction of a carat can affect the price of a diamond by hundreds or thousands of pounds, however, it should be remembered that this is a measure of rarity as opposed to just quantity. Large diamonds are rarer than small diamonds; that is why they are more valuable, not just because they are bigger. That is why a two-carat diamond will be worth more than twice as much as a one-carat stone.

There are also ‘magic numbers’ where the price shoots up dramatically. A one-carat diamond will cost significantly more than a 99 point one, even though they are almost indistinguishable regarding size and appearance. 50 points (half a carat) and 75 points (three-quarters of a carat) are similar symbolic thresholds.

While the four C’s are the universal measures of a diamond’s objective value, we encourage our clients to look beyond these and to choose a diamond that is right for them. However, understanding this grading system is the only way to know how the value of a diamond is arrived at, and as such is an essential part of the buying process.


Diamond clarity is determined by the absence of surface blemishes like nicks and scratches and, more frequently, internal features called inclusions. These are often tiny crystals trapped inside a diamond when it was formed. The highest grade of clarity is flawless. That is when a diamond has no blemishes or inclusions visible to an experienced grader at 10x magnification. Such diamonds are so rare that many professional jewellers have never encountered one.

For those diamonds that are less than flawless, clarity grading takes into account not just the number of inclusions but how big they are, what form they take, their colour and position. An inclusion on the underside of a cut diamond would not be as significant as one on the top. That means that each diamond is assessed on its own merits.

11 grades of clarity exist. At the top are flawless and internally flawless. Next are two grades of diamonds considered to be very slightly included (VVS1 and VVS2), two grades of very slightly included (VS1 and VS2), two grades of slightly included (SI1 and SI2) and three grades of included

diamonds (I1, I2 and I3). Only those diamonds at the bottom of the scale have inclusions that can be seen by the naked eye. At Harper Tait, we only buy diamonds that range from flawless to SI1.


The rarest and most desirable diamonds are those that are as close to colourless as possible. Most have yellow or brown tints, and subtle differences in colour can dramatically affect a diamond’s value, regardless of other factors. Diamonds are graded for colour from D to Z, with grades D-F being considered colourless, G to J near colourless, K to M faint, N-R very light and S-Z light. At Harper Tait we only use diamonds graded D to F: that is, those that are officially rated as colourless.

Another key consideration is fluorescence. Roughly 35% of all diamonds emit a fluorescent glow when exposed to ultra-violet (UV) radiation, for instance from sunlight or fluorescent tube lights. The most common colour for this fluorescence is blue, and if a diamond has a yellow tint, then a blue fluorescence can cancel this out. A strong fluorescence can also give a diamond a cloudy appearance which reduces its worth.


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