The main rules of 4C

Grading diamonds based on professional expertise, grew in response to the rapidly growing market for diamonds: the GIA (Gemmological Institute of America) established the Gem Trade Laboratory in the USA in 1935, the Gubelin laboratory in Lucerne issuing certificates since 1940, in 1961 the international organization, CIBJO was established to join producers and sellers of precious stones, jewels and pearls, and in 1976 the Dutch Department of Certification established the Diamond high council (Hoge Raad voor Diamanten – HRD).

There are four basic parameters for evaluating and certifying diamonds: cut, carat, clarity and colour which have a significant influence on the price of diamonds.

These parameters are called the 4C.

Other parameters that can be in certificates are: size, fluorescence, internal and external characteristics, etc.

To better understand the evaluation of diamonds, we recommend the world-renown book by Verena Pagel Theisen “Diamond Grading ABC” which was translated into Czech by Engr. Ladislav Klaboch: Diamanty: Příručka hodnocení diamantů.

CUT

The modern brilliant cut is based on optical calculations focusing on maximizing the beauty of a diamond that comes as a result of its brilliance and dispersion.

The brilliant cut has a round outline and consists of a crown (32 facets + table) and a pavilion

(24 facets + a possible culet).

Examination of the quality of a stone´s cut is based on comparing its ideal proportions with the symmetry of a mathematical model. Deviations from such a model are known as cut quality and are divided into several grades:

Very good, good, fair, and poor. Some laboratories have the categories excellent or ideal cut for the best diamonds. The final state of a cut is called its finishing.

The effect “Heart and arrows“ occurs when the round diamond cut is of very high quality. One can then see the image of the above mentioned naming.

COLOR

The diamond is the only gemstone that occurs in all shades of colour but the most common colours (after cutting) are from white to yellow. An international rating system from “D” to “Z” was developed for these basic colours. The whiter the shade of the stone, the closer the mark of the stone is to the beginning of the alphabet.

Colour specification must be performed by experienced professionals by using a set of sample stones and comparing them with a given stone. Carefully controlled lighting conditions must be used for example, normalized light of 5000/5500 K (D55) and a standard procedure for the classification of the examined stone to the right colour grade.

INTERNATIONAL COLOR GRADING OF HRD, GIA, CBJO

(1)       (2)
Exceptional white+ D RIVER
Exceptional white E
Rare white+ F Top Wesselton
Rare white G
White H Wesselton
Slightly tinted white I Top Crystal / Crystal
J
Tinted white K Top Cape
L
Tinted color  (3) M From Cape to Yellow
1 – faint yellow, 2 – very light yellow,

3 – light yellow, 4 – yellow

Z
Fancy Diamonds
(1) GIA Terms

(2) Transitional use until 1990

(3) Optional subdivision

Fancy color are diamonds with intensive coloring. 

Fluorescence

When the diamond is illuminated by ultraviolet light it can emit visible light on different levels (none – slight – medium – strong). These diamonds are fluorescent.

Degrees: none – slight – medium – strong

CARAT

The weight of a diamond is always given in carats and the international abbreviation, ct is used. The weight is indicated in certificates with 2 decimal places. Measurement is done with an accuracy of 3 decimal places. The second decimal place can only be rounded up if the third decimal place is 9.

1 ct = 200 mg = 0.2g

According to the strict defined parameters of a brilliant cut, it is possible to count the weight also by measuring the basic proportions of a brilliant-cut diamond, e.g. with the diameter (at least two axes perpendicular to each other) and height.

Estimated density [ct] = (girdle circumference) 2 × height × 0,0061.

The carat unit comes from middle age marketplaces where carob seeds were used as a weight unit for trading with precious stones because of their reputation for having a uniform weight.

CLARITY

The clarity is given by the number of foreign substances included in the diamond.

Such in-homogeneities may be internal in which case they are called inclusions (black “carbon“, white “ice“, cracks, etc.). They arise during particular phases of crystal formation. Further, defects may be of an external character (scratches, extra facets), which may be caused through cutting and polishing or defects that couldn´t be removed during processing.

The size and the placing of these inclusions are key factors for the process of grading clarity. A standard diamond grading loupe of 10x magnification is used for measuring clarity.

THE INTERNATIONAL SCALE OF DIAMOND CLARITY

LC – Loupe Clean or IF – Internal Flawless

If the diamond is transparent without any visible inclusions, it is loupe-clean.

VVS* (Very Very Small) (VVS 1, VVS 2)

Very very small inclusions hardly detected by a diamond grading loupe.

SI* (Small Inclusions) (SI 1, SI 2)

Small inclusions easily found by a diamond grading loupe but invisible on naked eye from the table side.

P I (Pique)

Medium inclusions which can be easily located by a loupe but hardly seen on naked eye.

P II (Pique II)

A big number of large inclusions easily detected on naked eye.

P III (Pique III)

Big and numerous inclusions easily detected on naked eye and influencing the brightness of the diamond.

*Other degrees of the grading VVS, VS and SI and their two subcategories can be used only for grading of 0.47 ct diamonds and bigger.

Using terms like “pure”, “pure to eye”, “commercial pure” or other confusing expressions are not allowed.

Properties of Diamonds

Hardness

The word diamond is derived from the ancient Greek ada-mas - invincible. Diamond is the hardest known natural material. It reaches the highest degree of hardness, 10, on the Mohs scale of hardness. The group of corundum minerals (e.g. ruby, sapphire) reaches a hardness degree 9. Although there is a difference in just one level, a diamond is 140 times harder than corundum. This difference is the same as between degree 1 and 9.

Density

Diamonds have a specific mass of 3,51g/cm3. Thanks to this property diamonds settled in river beds and created so called alluvial deposits. The specific mass is therefore the basic indicator in determining the authenticity of diamonds.

Colours

The white colour around us is actually composed of a whole scale of colours from red to violet. Depending on what range of colour is absorbed or radiated, creates the resulting colour. Almost all diamonds contain atoms of other elements. Atoms of nitrogen are most common and they cause a yellow colouring that we can see (e.g. it absorbs the blue spectrum). Boron causes a blue colouring (absorbs the yellow spectrum). Other colour alternations are caused by different mechanisms, for instance by deformation of the crystalline lattice – pink diamonds, or radiation – green diamonds etc. Diamonds without any other element are completely colourless.

Fluorescence

This property of diamonds was first utilized in the 60s of the 20th century for grading of diamonds. Every diamond exposed to X-rays radiates and can thus be separated from other rocks. The fluorescence of diamonds is usually blue but can also be white, violet, yellow, green or orange.

Transparency, Openness, Translucence

According to transparency we distinguish diamonds from absolutely clear to completely opaque. Only the best stones are set in jewellery. Other stones are used in industry where they play an indispensible role.

The refractive index

The refractive index in diamonds reaches a specific value of 2,417 and lends the diamond its unique optical phenomena, which also serves for determining its authenticity.

Game of Light - Brillance scintillation

The number of reflections, when moving a cut stone or the source of light, is called sparkling brilliance. The number of light reflections depends on the number of facets and on their proper size.

Thermal conductivity

Diamonds possess 6 times higher thermal conductivity than copper. This property is used for both testing diamonds and their application in nanotechnologies.