Diamond Guide

Diamonds – a guide to quality and value.

The most precious and enduring of all gemstones, diamonds were formed billions of years ago. They are created when carbon is put under immense pressure and temperature, deep within the earth at distances of 100 miles or even greater. Only a small number make the journey to the earth’s surface, fewer still are of a quality that can be made into a precious piece of jewellery.

Where do diamonds come from?

Primary deposits are found in a volcanic rock called “kimberlite”. Kimberlite sites can be found all over the world but only a small number contain the precious diamond crystals. Secondary deposits, also referred to as alluvial, were formed as a result of erosion of the material from the primary deposits and contain diamonds that have travelled some distance from their original source.

There are several reasons why diamonds command such high prices. Even though world diamond production has tripled since 1980, diamonds remain a scarce resource. About 6,400 kimberlite pipes have been discovered in the world, of those about 900 have been classified as diamondiferous, and of those less than 1%, just over 30, have been economic enough to diamond mine. Only a tiny percentage of crystals will be of gem quality.

Diamond mines tend to be in very inhospitable environments making the extraction cost high. Enormous investment and technical skills are necessary to construct maintain and operate the mine. In open pit and underground mines, the ore is crushed to uncover the diamonds. Coastal mining involves the evacuation of tonnes of sand to find diamonds. Undersea mining entails drilling into the seabed to recover diamond-bearing gravels.

In today’s conservation-minded world mining companies have to guarantee to restore the sites back to their natural state once extraction is complete so adding to the final mining costs. This is all before the diamond is even cut!

Diamonds are graded and valued based on their own unique characteristics, these are known as the 4 C’s. Diamond grading is a subjective judgement made by an experienced grader against an agreed industry standard.

The 4 C’s


Cut is the only diamond characteristic directly influenced by man. Cutting a diamond is a highly skilled craft and whilst technology has developed some aspects of the profession, it is still primarily a ‘hands on’ trade. In their natural uncut ‘rough’ state, diamond crystals could be mistaken for pebbles. Rough diamonds are sorted into many different categorises, but there are six basic types of rough; stone, shape, macle, cube, flat and cleavage.

The basic shape will have a direct influence on what ‘cut’ the cutter chooses to get the maximum yield (size) and brilliance from the crystal.

  • Round
  • Baguette
  • Princess
  • Marquise
  • Oval
  • Pear

Today there are many different cuts, but each have ideal mathematical proportions. These have been developed through the years to give optimum dispersion of light, which a cutter will strive to meet with precision and minimal loss of yield that the crystal allows. A stone which is cut too shallow or too deep will lose light through the side or bottom and the overall brilliance and fire of the stone will be affected.


Most diamonds appear colourless to greenish-yellow. The closer to colourless, the rarer and more valuable the stone. Stones are graded by judging the amount of saturated colour within a stone.

D – Exceptional White

E – Exceptional White

F – Rare White

G – Rare White

H – White

I – Slightly tinted White

J – Slightly tinted White

K – Tinted White

I – Tinted White

M – Tinted Colour

N – Tinted Colour

O – Tinted Colour

P – Tined Colour

Q – Tinted Colour

R – Tinted Colour

S-Z – Tinted Colour

Colour grading is carried out in a neutrally coloured environment, preferably grey, with no sunlight and under a daylight lamp of industry standard. The stone is observed upside down, perpendicular to the pavilion side. They are normally judged against an industry standard master set of stones which represent each colour grade.

Diamonds with a pure spectral colour are extremely rare and are called ‘fancies’. These include pink, blue, yellow, green and orange. The hue and brightness as well as saturation are taken into consideration when grading coloured diamonds.


As a diamond forms, irregularities in the crystal structure can occur, or small particles of other minerals can get trapped within the diamond. These are referred to as inclusions or the ‘fingerprint’ of the diamond. A grader will use a x 10 magnification chromatic loupe to grade a diamond.

FI/IF (flawless/Internally flawless)

VVS1 – VVS2 (Very, very small inclusions)

VS1 – VS2 (Very small inclusions)

SI1 – SI2 (Slight inclusions)

I1-I2-I3 (Inclusions visable to the naked eye)

Consideration will be taken of the size and nature of the inclusion, how contrasting it is within the diamond and how easy it is to find. These factors will determine which grade to assign to the stone. The fewer, the smaller, the less contrasting and the more difficult to find, the more valuable the stone.

Carat –Weight

Carat is a weight of measurement (1carat = 0.2g) and therefore is the only one of the 4C’s that does not require the expertise of a grader. The name is derived from the seeds of the Carob bean, which, because of their uniform weight, were used by early traders to measure stone weights. Most diamonds are smaller than 1carat (ct) so the measurement has been divided into points (pts) 0.50ct = 50pts.

0.25ct (4.1mm)

0.50ct (5.15mm)

1.00ct (6.5mm)

1.25ct (7.0mm)

1.50ct (7.4mm)

1.75ct (7.8mm)

2.00ct (8.2mm)

2.50ct (8.8mm)

3.00ct (9.35mm)

It is important to remember that the depth increases, as well as the diameter, when moving up the carat sizes. Therefore looking down on a 1ct diamond, although significantly bigger, it does not visually look twice the size of a 0.50ct. In fact there is only 1.3mm difference in diameter.


Diamonds are a powerful symbol of love and eternity and selecting a diamond is an emotional and exciting event. It is therefore important to have confidence in your jeweller. The Steensons are renowned for their professional customer service, quality craftsmanship and designs as individual and unique as the diamond within.

Steensons have four members of staff qualified in diamond grading. The Steensons and all their suppliers strictly comply with the Kimberly Agreement ensuring that all our diamonds are ethically produced and conflict free.