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The play of colors

18 june 2018

Color is the second measurement of the ‘4C’ system used by experts for the polished diamond grading. Talking about this parameter, the jewelers imply the presence or absence of a hue in a classic colorless – or so called ‘white’ - diamond. Part of stones mined have a marked hue – they are called ‘color or fancy color’ diamonds, but some other methods are used to assess them.

Ideally, completely colorless rough diamonds contain no impurities (their content is under 0.001%), it means they consist of pure carbon. Such stones are rather rare in nature and that is why they are very valuable on the jewellery market. Besides, due to the lack of irregularities in the structure a white diamond has the best internal color reflection, which results in better glittering and sparkling.

Most of the polished diamonds have various color intensity. Diamonds formed due to the crystallization of the carbon molecules under high pressure deep in the Earth, and in this process, the atoms and molecules of other substances penetrated into the crystals and deposited as impurities. The color of gems is attributed to impurities and structural irregularities. the diamonds of similar colors are often mined at the same deposits, which can be explained by the mineralogical peculiarities of the area.

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Figure 1. Polished diamonds of various hues – from the colorless to the yellow one. D, K and Z color grades as per the GIA grading system.

Nitrogen is the key impurity in diamonds, and the diamonds can be sorted out into several categories based on the nitrogen content.

Type 1 are nitrogen-containing diamonds with the N-concentration of up to 0.25%. if the nitrogen atoms form a pair, they have no influence on the crystal color – these are Type IaA diamonds. And the impurities consisting of the conglomerates made by these pairs give the color a yellow or brown hue – these are Type IaB diamonds. About 98% of all the gem-quality diamonds mined are Type IaA, Type IaB diamonds or their combination. The experts call this group as the ‘Cape’ series - named after the South African province rich in gems of this type. Such diamonds absorb light both in the infrared and ultraviolet regions and their fluorescence is in the blue spectral band.

I should make a digression: Fluorescence is the ability of a diamond to glow when it absorbs radiation that is in the ultraviolet region of the spectrum. Many gems show so weak fluorescence that is seen under the special lamp in a dark room only. However, individual gems show fluorescence even when exposed to the usual daylight. For the diamonds of higher color category, the fluorescence is rather a defect, but in yellowish stones, the strong blue fluorescence can neutralize this hue so that in the daylight, the stones would look as colorless ones. Fluorescence is not included in the ‘4C’ diamond grading system but can influence the price for a stone. For example, there is a demand on the US market for colorless diamonds with scarcely perceptible hues that is why they are slightly more expensive. In Europe, on the contrary, such polished diamonds are in lower demand and their price is lower. For the Russian market, the fluorescence most commonly is of no importance.

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Figure 2. fluorescence: This is how the same polished diamonds look like in the ultraviolet and daylight illumination. The fluorescence grading scales are: none, faint, medium, strong, very strong.

Back to the classification. If nitrogen atoms are all over the crystal (without making atom pairs or atom pair groups, which is important), the mineral often becomes bright-yellow – canary – colour, sometimes it has a cognac (brown) color and refers to a color diamond group. This group makes about 0.1% of the known gem-quality diamonds. By the way, almost all HPHT synthetic diamonds contain nitrogen impurities of this type and specific properties of fluorescence. Most of the devices designed to detect the lab-grown diamonds are based on this and the devices work rather well and do not reject natural diamonds – the statistics show that just 2% of the natural diamonds are rejected and need additional testing.

Type II diamonds are those lacking the measurable quantities of the nitrogen impurities. The type IIa diamond group accounts for 1.8% of all the gem-quality diamonds that have no impurities at all. These stones are colorless, permit light through perfectly well and are very valuable. Yellow, brown, orange, pink and violet colors can be attributed to the structural irregularities that occur in them due to the plastic deformation in the process of their formation. Many large famous rough diamonds (like the Cullinan and Kohinoor ones) are Type IIa stones. The synthetic diamonds manufactured by CVD - chemical vapor deposition are also Type IIa stones.

Type IIb diamonds that make 0.1% of the fine diamonds are among the rarest ones. They contain significant boron impurities that make them light-blue or blue-grey color (Type Ia diamonds can have the blue-grey hue but it is due to structural peculiarities and not due to impurities). By the way, Type IIb diamonds are the only ones with the semiconductor properties.

Table 1. Diamond classification based on the nitrogen content.

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Green diamonds stand apart - their color is attributed not to impurities but to the structural changes caused by the radiation in the process of the diamond formation. These diamonds are exceptionally rare.

When grading colorless diamonds using a ‘4C’ system, the expert should determine exactly the polished diamond color intensity in the standard lighting conditions using color standards. Nowadays, master sets of cubic zirconium are often used as a standard instead of a diamond master set as they are ten times cheaper than the diamond standards and are practically of the same quality.

The GIA master set is the most popular one widely used – in Russia as well - to grade color in diamonds. It is subdivided into groups and starts with absolutely colorless polished diamonds designated by a Latin ‘D’. Then the color changes and the yellow tint intensity increases and the final group has a distinctive light yellow color and designed as ‘Z’. The same master set is used to grade the diamonds of brownish, greyish and greenish hues, because it is the color intensity that is important and not the color itself. The diamonds of the colors beyond ‘Z’ are considered to be fancy-color ones. Often, retailers do not carry colorless diamonds in the range lower than ‘J’ due to a low demand for them.

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Figure 3. The GIA master set used to grade the color in diamonds.

Grading is done as follows: an expert puts polished diamonds face down in a row on the paper near its central bent. There should be no dark areas on the working surface, and the lighting should be soft to exclude reflection and diamond sparkling that interfere (by the way, special jewellery lamps with daylight temperature are produced for color grading). The expert doing color grading should look at the stone perpendicular to the polished diamonds surface. In such a positon, it is possible to quickly detect slight color tints of each diamond if to look very attentively. Then comparing them with the master set, the expert gives the final characteristics of the color.

When a diamond is set in a jewellery piece, grading is given with 1-2 level accuracy, because the color of the mounting influences much on the color perception. Here lies some jewellers’ artfulness – those stones that look yellow in white gold mountings, will look practically colorless in yellow gold mountings; that is why yellow diamonds are often set in yellow gold mountings. So, platinum and white gold require top-color grade diamonds.

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Figure 4. J-color stone looks more colorless when set in yellow gold.

The Russian color grading system is governed by state standard GOSТ R 52913 – 2008, that has 4 color gradations from small-size polished diamonds with 17 facets, 7 gradations for the rest diamonds weighing up to 0.29 ct, and 17 groups for 0.3+ct gems. The diamond GIA characteristics and their respective GOST characteristics are given in Table 2.

As for the diamond price, the color influences the value almost lineally for the small-size stones (under 1 ct), and it decreases with every hue gradation. As for the large-size stones, the influence of the color on the diamond price is especially marked for the top quality diamonds. The comparison of the round Ideal diamond prices of the comparable size (1 ct) and clarity (IF, VVS2, SI2) and available in April 2018 is given in Figure 5.

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Vertical axis: Price, USD; horizontal axis: Color

Figure 5. The polished diamond price vs the color of the diamond. The data provided by Blue Nile, Brilliance, Brilliant Earth, Ritani, James Allen was compared.

Table 2. The GIA color scale for the colorless polished diamonds.

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And a little bit about the colored diamonds. They fall outside the GIA’s grading scale and are graded “by measuring hue, saturation, light-to-dark tones (intensity), and the diamond’s uniformity of color distribution”. Unlike regular diamonds, for which a carat weight, clarity, color and cut (the 4Cs) are equally important, the color is the most important characteristic for these diamonds. The 27-color scale is used to describe the hue.

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Figure 6. The polished diamond color-grading scale (Source: Asteria Diamonds).

Each color has nine saturation gradations. The key ones are faint, light, fancy light, fancy, fancy intense, fancy deep, fancy vivid. The value of a fancy color diamond jumps with the increase in the tone saturation. Fancy vivid color diamonds are really the most expensive and rare color ones followed by the fancy intense color gems.


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Figure 7. Tone and saturation scale for color diamonds (Source: Asteria Diamonds).

There exists another kind of gems like black diamonds. They became black due to the graphite and hematite impurities, are not transparent and reflect the light from the surface only. Moreover, they have a special diamond structure – carbonado - different from that of the rest diamond types and consisting of small-grain irregular-shaped parts (fragments) of crystals. This explains their being fragile and difficult in cutting. Strictly speaking, until recently, black diamonds were considered a rejected material and were not used in the jewellery manufacture. The jewellery house de Grisogono made them fashionable when they launched a collection of rings, plaques, earrings, cuffs and watches decorated with black diamonds. Though they gained worldwide recognition, the price for the black diamonds is much lower than that for translucent diamonds.

Translucent black diamonds are rare in nature and the matter is in the abovementioned structural irregularities and not in impurities. Such stones are sold at a fancy price and at the auctions only.  

Apropos: Historically, various systems were used to designate the polished diamond quality. They included I, II, III и A, B, C; then – АА and ААА for rough diamonds of the quality higher than А. Their color often was described by metaphorical terms like ‘First Water’, ‘River’ or, for example, ‘Cape’ to describe the yellowish diamonds from the Cape of Good Hope. Later on, when the Gemological Institute of America (GIA) was established and the GIA International Diamond Grading System was developed, one of the tasks was the clear-cut dissociation of the new scale from the existing ones. That is why the color-grading scale to assess the color in Gia starts from ‘D’ to distinguish from A, B and C that were used in other less consistent scales.

Olga Tretyakova, Rough&Polished