Diamond is an amazing crystalline material. Perhaps it’s most supreme property is that it is the hardest substance known. Alone amongst the familiar gems, diamond is composed essentially of a single chemical element, the lightweight element carbon.
Commercially, diamond is the most important of all gems. It is estimated that diamonds account for approximately ninety per cent of value of gemstones purchased throughout the world.
All diamonds which we see as gems grew in the earth’s mantle. The evidence for their mantle origin lies in the diamond impurities, pressure experiments and geological research.
Like any other mineral, during growth, diamond retains impurities from its surrounding rock, ranging from single atoms to complete mineral crystals. Those which can be seen under an ordinary gemmological microscope are called inclusions.
Diamonds are formed from carbons at between about 900°C and 1300°C, at pressures of over 45 kbar and at depths of 120 km below the earth’s surface.
Diamonds derived from such carbon are at least three billion years old. Diamonds also grew within mantle eclogite rock where the carbon may have derived from early micro-organism that lived in the mud of the sea beds. Diamonds derived from such carbon are less than three billion years old.
The 4 C's
Gemstones value is based on a combination of features, sometimes called value factors. As with gemstones, diamonds with certain qualities are more rare and more valuable than diamonds that lack them. Without a systematic way to evaluate and discuss these factors, there would be no way to compare one diamond to another.
Diamond professionals use a special set of four value factors to describe the classify diamonds:
Colour – The grade of colour, either the relative ‘whiteness’, or the rarity and desirability of fancy colours
Clarity – Clarity, or relative freedom from inclusions
Cut – The style and quality of cut
Carat weight – The weight of the stone
When used together, they describe the quality of finished diamond, which is directly related to its value.
Diamonds are weighted in metric carats:
The weights of loose stones are measured on carat scales
Carat is denoted by ‘ct’
1 carat = 0.2 gram
5 carat = 1.0 gram
The name ‘carat’ is derived from the name of the carob or locust tree (Ceratonia siliqua) found in Mediterranean lands.
The dried seed of this tree were once used by traders as unit of weight for gems because the seeds have remarkably consistent which is around 1/5 gram.
The majority of polished diamonds are graded for their lack of colour.
If one diamond can be shown to have a ‘better colour’, i.e. less colour, than another of the same weight, clarity and cut, then it will have a higher value.
If a stone has noticeable and attractive colour, it is termed a fancy coloured diamond. Fancy coloured diamonds have their own pricing scale; most are valued for their depth of colour rather than their lack of colour.
Most polished diamonds appear ‘colourless’ and are termed ‘white’. The Maajority of these, however, exhibit just a tint of yellow, even though to the untutored eye they are all more or less colourless. The range of yellow tints has been called the ‘Cape Series’ , as colourless to pale yellow diamonds were commonly found in the Cape region of South Africa when this term was first used. Diamonds also occur with tints such as brown, grey or green.
Each colour grade represents a ‘spread’ of range of colour, not just one point along the colour series.
The point of division between each colour grade in any colour grading system is an entirely arbitrary point.
The continuous colour range is thus divided into selected ranges in order to simplify the assessment of quality. It enables one person to describe a stone to another person using fixed reference points that are familiar to both.
GIA Colour grading scale:
D to F: Colourless G to J: Near colourless K to M: Faint yellow N to R: Very light yellow S to Z: Light yellow
The clarity of a diamond refers to a stone’s freedom from inclusions and blemishes. Clarity is graded on a scale of ‘flawless’ to ‘imperfect’, as determined by an experienced grader using 10x magnification.
The grading characteristics in or on a diamond are:
Internal characteristics – those features which are either totally enclosed within the stone or extend into the stone from surface (inclusions).
External characteristics - those features which are found on the surface of the stone but which do not penetrate into the stone (blemishes).
Definitions of clarity grades according to internal defects:
FL – Flawless - Shows no inclusions or blemishes under 10 x magnifications.
IF – Internally Flawless- Shows no inclusions and only insignificant blemishes under 10 x magnifications.
VVS- Very Very Slightly Included – Contain minute inclusions that are difficult for experienced grader to locate at 10x magnification.
VVS1 – Inclusions that are extremely difficult to see.
VVS2 – Inclusions are very difficult to see.
VS-Very Slightly Included
VS1 – Inclusions are difficult to see.
VS2 – Inclusions are somewhat difficult to see.
SI-Very Slightly Included
SI1 – Inclusions are easy to see.
SI2 – Inclusions are very easy to see. (may be seen with the unaided eye)
I or P –Imperfect or Pique
I1 0r P1 – Beauty or durability are somewhat affected.
I2 0r P2 – Beauty or durability are seriously affected.
I3 0r P3 – Beauty or durability are very seriously affected
Cut of a Diamond:
Cut is categorized by the following:
Shape – The outline of the diamond combined with the facet distribution;
Proportions – The relationship between various parts of a diamond and the girdle diameter;
Finish – The exactness of its shape and the arrangements of the facets. Also the quality of the polish.
The most popular cut for diamond is the standard ‘round brilliant’. The 57 facets of the ideally-proportioned brilliant cut are polished at specific angles relative to one another. The standard brilliant is round in outline with 33 facets on the crown and 24 facets on the pavilion. The base of the pavilion terminates in a point which is called the culet. This may be polished as a small facet bringing the total number of facets to 58.
Brilliance – is the degree of brightness resulting from reflection of light by gemstone when viewed through the crown facets. It is made up of light reflected from the pavilion facets and from the surfaces of the crown facets.
Fire – is the breaking up of white light into its spectral colours when white light passes through the inclined facets of a cut material. Diamond, strontium titanate and synthetic moissanite are noted for their great amount of fire.
Symmetry of a Diamond
The symmetry of a stone can be split into major and minor symmetry faults as below.
Major symmetry faults:
Out of round girdle outline (off-round)
Wavy girdlr or tited table.
Minor symmetry faults:
Any symmetry defect mentioned above, but not severe.
Facets fail to meet prperly (facetes not properly pointed)
Misalignment of crown and pavilion facets.
Table is not a regular octagon.
Symmetry is classified as Excellent, Very Good, Good, Fair or Poor.
Minor symmetry points have far less impact than have the major symmetry faults on the grade given.
Excellent - No symmetry deviations
Very Good - No visible symmetry deviations seen
Good - Only minor symmetry deviations seen
Fair -Visible symmetry deviations seen
Poor - Major symmetry deviations seen
NOTE -An Excellent symmetry grade is only possible on very close inspection either with the aid of a microscope or with a proportion scope.
Polish of a Diamond
Quality of polish is affected by:
Polishing lines - parallel lines on the surface of the stone produced during polishing (these will be oriented in a different direction on each facet)
Burn marks caused by poor polishing.
Polishing is defined as being Excellent, Very Good, Good, Fair or Poor
Excellent - No polish deviations
Very good - No polish imperfections found or difficult to find under 10x magnification.
Good - Polish imperfections are fairly hard to find under 10x.
Fair - Polish imperfections are easy to find under 10x and may be seen with the unaided eye.
Poor - Polish imperfections are easy to find under 10x and are easily seen with the unaided eye.
NOTE - An Excellent polish grade is only possible with the aid of a microscope.
The quality of polish is an important part of a diamond’s cut. A good polish is required to show maximum brilliance, sharp scintillation and clear dispersion.
It also demonstrates the skill and care taken in the cutting process.
Fluorescence of a Diamond
Individual diamonds have different responses to ultraviolet radiation. Some diamonds are inert and do not fluorensce at all. However when diamonds do fluresce, they usually do so more strongly under long-wave ultraviolet (LWUV) light, and with a blue glow, varying in intensity from faint to very strong. Yellow, green, orange and other fluorescent colours also occur in individual diamonds.
Blue fluorescence can mask the yellow tint of a diamond and thus the diamond will appear to be a better colour grade than it would have in a light source free from UV light. Thus it is important to ensure controlled light conditions when colour grading a diamond.
Fluorescent is graded under long-wave ultraviolet light and is given a grade according to the intensity of fluorescence seen: i.e. None, Faint, Medium, Strong and Very Strong.
A stone with very strong fluorescence and high colour grade for example a D or E coloured stone of VVS clarity, may be discounted in value by about 5 to 15 %, depending on the diamond dealer. Conversely a fluorescence stone of a lower colour such as L, of VVS clarity, may have an increase of a couple of percent on the price. Percentages vary considerably depending on the stone, the diamond dealer, the circumstances and the purchaser.