Diamonds – A Guide

Emily trained with the Gemmological Association of Great Britain (GEM A) and enjoys sharing her notes and experience in a guide to explain why diamonds are the King of gemstones.  Jump straight to the section you need or read the guide here. 

Crystal Form

Chemical composition
Crystal system
10 (the highest)
Yellow, brown
Fancy, rare
Green, pink, blue, black
Fancy, very rare
Red and purple

The crystal form of a rough diamond depends upon such variables, such as temperature, pressure, growth rate and trace elements.

The Octahedron form is the most common habit for diamond.  Some complex forms do occur, including dohecahedron and the triangular twinned crystals called ‘macles’.

Grading Systems – the 4 C’s:

Carat (ct) weight
The weight of the stone
The grade of colour, the relative ‘whiteness’ and the rarity and desirability of fancy colours
Clarity, or relative freedom from inclusions
The style and quality of cut
Carat weight
All other qualities being equal, the heavier the diamond, the higher its price per carat
This follows a non-linear scale: larger diamonds are proportionately much more valuable. ‘Desirable’ weights, for example, are 0.50ct, 1ct, 2ct etc; a 0.95ct diamond is worth less per carat than a 1.01ct diamond of the same quality.
For loose stones, weights are measured on carat scales
1ct = 0.2grams, 5ct = 1 gram
For intermediate weights, each carat is split into 100 points (pts):
1.00ct = 100pts, 0.50ct = 50pts, 0.01ct = 1pt






M - Z

‘Colourless’.  These grades are judged by ‘lack of colour’ and increased brightness.

Slight tint of colour

Tint of colour


This the point where most people start to perceive a draw of colour in a diamond


A stones freedom from inclusions. Clarity is graded on a scale of ‘flawless to imperfect’ as determined by an experienced grader with a 10x lens.

All characteristics found in or on a diamond can be divided into either:

Internal characteristics – features totally enclosed within the stone or extend into the stones from the surface (inclusions).

External characteristics – features found on the surface of the stone but do not penetrate into the stone (blemishes).

Factors affecting the clarity grade:

Number of inclusions
Greater number, poorer the grade
Size of inclusions
Larger the inclusions, less brilliant the stone and hence the poorer grade
Position of inclusions
More central the inclusion(s), the greater visibility and hence the poorer grad
Brightness of inclusions
Darker the inclusion, greater its visibility and hence the poorer grade
Nature of inclusions
A faint ‘cloud’ (group of very small inclusions) will have a lesser effect on clarity than a dark crystal of same size and position if other factors are equal.


Flawless (FL)
Shows no inclusions or blemishes
Internally Flawless (FL)
Shows no inclusions and only insignificant blemishes
Very Very Slightly Included
Minute inclusions difficult to locate with a 10x lens
Inclusions are extremely difficult to see
Inclusions are very difficult to see
Very Slightly Included
Contains minor inclusions; small clouds, feathers
Inclusions are difficult to see
Inclusions are somewhat difficult to see
Slightly Included
Contains noticeable inclusions, only with a 10x lens.
Inclusions can be seen
Inclusions can be easily seen
Imperfect / Pique
Noticeable inclusions with a 10x lens, which can be seen face up to the naked eye, can affect the diamond’s transparency and brilliance.
I1 / P1
Beauty and durability are somewhat affected, can be seen with the naked eye.
Beauty and durability are seriously affected, are seen with the naked eye.
Beauty and durability are seriously affected, easily seen with the naked eye.


Cut is categorised by the following:
Outline of the diamond. Symmetry of this shape is also important
Relationships between sizes and angles of various facets (also termed as the ‘measurements’).
Describes the quality of the finish of a diamond


Very good
No visible symmetry deviations
Only minor symmetry deviations seen
Visible symmetry deviations seen
Major symmetry deviations seen

The ideal proportions of a diamond:

In designing the brilliant cut, facets are angled to maximise brilliance and fire. Therefore, the angle between pavilion facets and the girdle is most important as it controls the production of brilliance by reflecting rays of light from the back facets. This must be correct to produce maximum internal reflection.


Very good
No polish imperfections found / difficult to see with a 10x lens.
Polish imperfections are fairly hard to find under 10x lens.
Polish imperfections are easy to find under 10x lens and may be seen with the unaided eye.
Polish imperfections are very easy to find under 10x lens and easily seen with the unaided eye

Occurrences & Localities

Diamonds are mined either from pipe-like volcanic bodies of kimberlite or from river and beach gravels where they accumulated after erosion from these pipes.

The alluvial deposits of India and Borneo were the only known sources of diamond from classical times until the eighteenth Century. The important Brazilian fields were discovered in about 1725.

The alluvial and kimberlite pipe deposits of South Africa were discovered in 1866/67 and resulted in much larger quantities of diamonds reaching markets.

Siberia began mining during the 1850s and in the early 1980s Australia became an important producer of diamonds, especially pink diamonds from the Argyle Mine. In 1998 Canada also started mining diamonds.

Other important diamond-producing countries include: Angola, Botswana, China, Namibia, Sierra Leone, Tanzania and Zaire.


Olivine is an important mineral inclusion as well as: garnet, pyroxene, spinel and other opaque minerals such as chromite and magnetite. Feathers and cleavages are other common inclusions.


The fluorescence of diamond varies in colour and intensity, from inert to strong. In natural diamonds fluorescence maybe bluish-white to violet, green or yellow and rarely red.

Diamonds which exhibit a bright blue fluorescence displays a yellow phosphorescence, pink stones may show an orange fluorescence and phosphorescence.

Sunlight may also stimulate a bluish fluorescence in some colourless diamonds.

Treatments in Diamonds

Fracture filling

Surface-reaching fractures in diamonds may be filled with glass and even small stones may be treated in this way too.

Diamonds are placed in a chamber in which hot glass of low-melting point is forced into fractures so they become less visible and the clarity of the diamond appears to have improved. The treatment is not permanent and can be damaged by heat (by re-cutting) and acid (by ultrasonic cleaning).

The filler is sometimes of a yellowish-hue which made reduce the colour grade of the diamond. A very characteristic interference colour flash; typically a yellow-orange to purple flash in darkfield illumination and a blue to green flash in bright illumination. These flashes are quite distinct from the usual iridescent colours displayed by diamonds.

As this treatment is not permanent and hides many of the surface reaching fractures in the diamond, fracture filled stones are NOT clarity and colour graded because the filler can affect the grade and should be declared.

Laser Drilling

A laser-drilled diamond may have drill-holes filled with glass or epoxy resin to prevent the entry of dirt. This will also fill any surface-reaching fractures and a flash effect may be seen both in the laser drill and surface-reaching fractures.

Diamond Simulants

The best instrument to check diamond-set jewellery is a 10x lens, which ultraviolet fluorescence is a useful back-up for testing many stones or jewels at once.

The most common diamond simulant in modern jewellery is cubic zirconia (CZ).  Observation with the 10x lens will provide clues to the identity of a colourless stone.  A diamond’s high refraction, dispersion, reflectivity, supreme hardness and inclusions each contribute to the distinctive appearance of a cut diamond.

Synthetic moissanite is another simulant which has a slightly greenish or brown tint.  Coupled with its high dispersion and large double refraction, which is detectable with a 10x lens, aids in its identification.

Thermal and Electrical Conductivity

Thermal conductivity describes the rate at which a substance can transfer heat.  Diamonds feel cool to the touch due to their high thermal conductivity. 

A thermal conductance probe is a portable instrument used to distinguish diamond from its simulants as diamond has a higher thermal conductance, with the exception of synthetic moissanite.

Synthetic Diamond (man-made) Diamonds

Gem diamond can be synthesized by two methods: the ‘belt’ process and the BARS method.  Both use high pressure and temperature to create diamonds.

Distinguishing inclusions in a synthetic diamonds are metallic flux and ‘dust’ or ‘bread-crumb’ inclusions or hour-glass colour zoning. Their crystals are predominately grow as combined forms.  The actual shape of these synthetic crystals is controlled by the temperature, pressure and orientation of the seed crystal during the synthesis process.

Comparison of Diamond Simulants





UV Fluorescence





Variable – inert to strong blue or yellow

Glass (paste)

Vitreous to dull

Low to high




Bright vitreous

Medium to high

7 1/2

None – weak yellowish

Cubic Zirconia (CZ)

Bright vitreous


5.56 – 6

Weak yellow orange to yellow-green, variable

Synthetic moissanite

Bright vitreous

Very high

9 1/4


Many factors go into selecting a diamond, however, this information somewhat relates to where your money is going.  After my years of selling diamonds the most important factor is also ‘gut’ feeling.  You will know when you have found the right piece of diamond jewellery for you or to give as a significant gift.

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