the Carat

Pretty things may come in small boxes, but what’s in those boxes better be big. That is, yes, size does matter. However – carat weight is not the same thing as size – how large a diamond appears is also dictated by other factors such as shape and cut, so it is important to understand carat weight beyond the value it is generally and often mistakenly ascribed.


Instead of grams or kilos, diamonds are weighed in carats (not to be confused with gold’s Karatwhich signifies purity). This simply denotes a measuring scale where each 1 carat = 0.2 grams (0.50 carat = 0.1 gram and 5 carats = 1 gram).

The term carat originates from the Greek and Arabic names for the carob tree – Keration in Greek and Qirrat in Arabic. The dried seeds of the Carob (or Locust) tree were once widely used by trading merchants as counterweights for weighing gold, diamonds, gemstones and pearls due to their relatively consistent weight and size. It is important to note however, that the term “carat” with reference diamonds is different to “karat” which is the value used for the purity of gold.

The Byzantine era used glass pebbles, based on carob seeds, for weighing coins, which weighed in at 196 mg, consistent with the average weight of an individual carob seed. However, their use eventually diminished as it was discovered that despite their visual uniformity, the seeds were not actually consistent in weight. Many attempts were made to standardise the measurement of gemstone weight and it was only in 1907, at the Fourth General Conference on Weights and Measures that the “carat” was adopted as the official metric measurement forgemstone weights.

In 1913 the United States officially accepted the ‘carat’ as the gemstone measurement, and in 1914 the UnitedKingdom and Europe followed suit. By the 1930s, the majority of the diamond and gemstone industry had agreed to the standardised measurement, which is still in use today.

Dimensions play an important role in the appearance of a diamond. In addition to the carat weight, the distance across the top of the diamond must also be taken into consideration. A common misconception is that half a carat is half the size of one carat. In fact, a half carat is half the weight of one carat, but the millimetre difference on a round stone is only 1.35mm. The average measurement for a 0.50ct stone is 5.00mm, while the average 1.00ct stone measures at 6.35mm. Download PDF guide here. (Hyperlink to PDF guide).

While carat weight may indicate a diamond’s size, the shape and cut of a stone also play a large part in determining how large or small the stone appears. An elongated shape such as the Marquise cut may appear larger than a rounded shape such as the round brilliant even if the two stones share the same weight. To read about diamond shapes in more detail, please click here (hyperlink to diamond shapes link to be advised by Fergus).

When comparing two stones of the same shape however, it is important to look at the cut grades and table and depth percentages, as shallower stones will tend to appear larger than deeper ones. Other aspects such as girdle width can also affect how large a stone appears, while not necessarily affecting the quality of the stone. To read about diamond cut in more detail, please click here. (As advised by Fergus, cut link)

The Cut

Far more than simply what shape a rough diamond has been polished into, the cut is the most important determiner of a diamond’s brilliance and light dispersion. Diamond cutting is an art requiring meticulous precision, and knowing how it all works is essential to making the right choice

The cut of a diamond has the single biggest impact on the overall beauty of the diamond and is, therefore, considered to be the most important aspect of the selection process. The better the cut the more the diamond sparkles (the greater the brilliance). Customers should try to maximise the cut specification selection within their own budget.
Please remember:

  • The cut has the largest impact on the brilliance (sparkle) of a diamond
  • The cut has the biggest impact on the beauty of a diamond
  • The cut is considered to be the most important characteristic of a diamond
  • The Fergus James gemologist team recommend putting “cut” as the most important variable in the selection process

The Colour

Colour grades actually refer to a lack of colour in a diamond, with the whitest grade being D, and that’s D for most desirable. However, even experts sometimes fail to tell the difference between some clarity grades, so unless it’s important for you what the letter on the certificate reads, you can often get away with lower grades.

the Clarity

Almost all diamonds have small impurities or “inclusions,” but they are not always visible to the naked eye. The size and location of the inclusions play a significant role in determining the price of a diamond so it is important to understand clarity grades in order to make the most suitable choice.

When diamonds are formed, deep underground and under extreme pressure and heat, imperfections in the crystal structure can form and mineral impurities become trapped inside the stone.

The size of these impurities and imperfections determine the clarity grading of a diamond. Diamonds without such impurities are very rare.

The grading scale starts from Flawless / Internally Flawless (FL/IF), Very Very Slightly Included (VVS1/VVS2), Very Slightly Included (VS1/VS2), Slightly Included (SI1/SI2) to Included (I1, I2 and I3). Fergus James does not sell diamonds below SI2 as they are not considered suitable for jewellery. Generally, diamonds below a VS2 grading are likely to have visible inclusions to the naked eye however this can be determined on a stone by stone basis.

The Certification

Diamond certificates are issued to confirm a stone’s technical characteristics, value and identity, but it is important to know and understand the differences between the grading bodies asthey operate on different standards.

When choosing a diamond you really need to take into account the certification of the diamond and which institute it has been graded by.

There are many institutes such as IGI, HRG, EGL and AGS however it is GIA (The Gemmological Institute of America) who have set the worldwide standards in diamond grading since 1931. GIA was the first of its kind and today stillexcels the other institutes in diamond grading. For example, when GIA grades a diamond as a D then the diamond is a D, when other institutes grade a D colour GIA may not consider the diamond a D as it may not be up to the right standard as the GIA. This also stands with the other 4c’s so it is always a good idea to familiarise yourself with the 4c’s prior to purchasing a diamond.

All diamonds at Fergus James are GIA certified diamonds, this is so we can ensure we’re giving our customers the very best according to international standards set by GIA.

the Shape

Elegant or eccentric, sparkly or subtle? From the ever popular round brilliant to the more unusual Marquise cut, diamonds come in all shapes and sizes and there is sure to be one to suit every taste and style.

Diamonds come in several different shapes. And whilst some people prefer some variety, the round diamond is by far the most popular: Eight out of ten engagement rings contain a round diamond. Whatever your taste or preference, however, Fergus James apply the same standards across our entire selection. We have access to one of the largest fine selections of diamond rocks in the world.

The basic diamond shapes are outlined below. Detailed information about each of these shapes will help you to make the selection that is right for you. The diamond shapes are ordered in terms of popularity from left to right.

need more Advice?

We are always here to help and advise, from ring sizing guides, to metal types. We are available to chat to 24/7. Please feel free to get in touch.