About Diamonds

About Thomas Belfast Diamonds

Diamonds are one of the most sought-after gemstones and have been used as adornments since ancient times. The hardness of diamond makes it resistant to scratching (although, not immune) and it's high dispersion of light, creating it's 'fire', mean that it is perfect for setting in jewellery. Diamonds have been used to symbolize a betrothal since at least the 15th century and wearing an engagement ring on the left hand was believed to ward off evil and attract good fortune - hopes which are still relevant today!

The Four Cs

Four characteristics are now commonly used as the basis for describing diamonds: carat, cut, color, and clarity. Diamonds of the highest qualities of cut, colour, clarity and carat are the rarest and therefore the most valuable. There are several international gemological associations, which provide grading reports, or certificates and these can be reassuring when deciding on the right stone. However, a diamond without a grading report is not necessarily of lower quality - it just means it doesn't have a certificate. Also, some grading laboratories are more reputable than others. At Thomas Belfast, we have built relationships with trusted diamond dealers and have access to a large collection of loose certified and non-certified diamonds.


The carat weight measures the size of a diamond. The carat is divided into 100 points, so that a 50 point diamond is the same as a half carat. The price per carat increases with carat weight, because larger diamonds are both rarer and more desirable. At milestone weights, such as 1.0ct, demand is high and the price of a diamond of around 0.95ct will be significantly less than a diamond weighing 1.05ct.

Total diamond weight (tdw.) is a phrase used to describe the total weight of diamonds when more than one gemstone is used. For instance, a pair of diamond stud earrings, may be described as having a tdw of 0.50ct, which means the diamonds weigh 0.25ct each.


Clarity indicates the extent to which a diamond is free from naturally occurring inclusions. These may be crystals of a foreign material or structural imperfections such as tiny cracks that can appear as dark specks or cloudy white. The number, size and position of inclusions can all affect the relative clarity of a diamond. Diamonds become increasingly rare at the higher clarity gradings. Only about 20% of all diamonds mined have clarity high enough to make them suitable for using in jewellery (the rest is used in industry). Of that top 20%, a significant portion are stones with one or more inclusions which are visible to the naked eye. At Thomas we work with diamonds between the grades of internally flawless (IF) and SI2.

Clarity chart:

FI – IF: Flawless/Internally flawless
V VS1 - V VS2: Very very small inclusions
VS1 - VS2: Very small inclusions
SI1 - SI2: Slight inclusions
I1 - I2 - I3: Inclusions visible to the naked eye


The finest quality of colour in diamonds is totally colourless and is graded as "D". The next grade has a very slight trace of color, however when set in jewellery these very light coloured diamonds do not show any colour and are graded as E or F. Diamonds which show very little traces of colour are graded as G or H colour diamonds. After this, the colour becomes rather more obvious, even to an untrained eye and slightly coloured diamonds are graded as I or over and may have a brownish tint. A diamond can be found in any colour in addition to colourless. Some of the coloured diamonds, such as pink can very rare and priceless. Brown diamonds, often described as Sherry, Brandy or Chocolate diamonds are increasingly being used by designers who want to use diamonds differently. At Thomas Belfast our white diamonds are of G colour and above.

Colour Grades:

D – E: Exceptional white
F – G: Rare white
H: White
I – J: Slightly tinted white
K – L: Tinted white
M – Z: Tinted colour (noticeably yellow)


The Cut is the only aspect of a diamond where man makes any input - the rest is up to nature. The cut of a diamond describes the quality of workmanship and the angles to which a diamond is cut and polished from rough stone to it's final proportions. The techniques for cutting diamonds have been developed over hundreds of years, but in 1919, mathematician Marcel Tolkowsky developed the round brilliant cut by calculating the ideal shape and number of facets to return and scatter light. Nowadays, the relative importance of carat weight may mean that a diamond is cut intentionally poorly to increase it's size, because there is a price premium for a diamond that weighs the magical 1.00ct. This stategy may not make the diamond appear any larger and greatly reduces the sparkle of the diamond as less light is reflected, impairing the diamond's fire and brilliance. At Thomas Belfast, we consider the cut to be the most important of the Four C's.