Monday, January 21, 2013

Color in Diamonds

Natural colored diamonds part of the National Gem Collection. © Smithsonian Institution. Photo: Chip Clark.

Do you always think of diamonds as being "white" (or, properly speaking, colorless)? Well... if so, think again! In nature, diamonds can be found in all the colors of the rainbow. The ones that are the most prized have an intense tint, referred in the gem market as a "fancy" to "fancy vivid" diamonds. Also, there are tricks to change, enhance or improve the color of a natural (or synthetic) diamond, but I won't get that much into these details on this blog post.

How does a diamond acquire its color?

Most minerals (remember, a gem is just a cut / faceted mineral!) are colored by chemical elements that produce color, for example:
- Copper gives a green, blue or red color,
- Manganese gives a red to orange to yellow color
- Cobalt gives a pink color
- Iron gives a blue, green, red or yellow color.

Diamonds do not contain such coloring agents. Instead, they can contain chemical impurities that don't have an intrinsic color, but their presence inside the carbon structure of a diamond induces some absorptions in visible light, resulting in the diamond coloration. These are called "color centers" or "point defects". Impurities are counted in part per million (one non-carbon atom for every one million of carbon atoms). The most common impurities in natural diamond are nitrogen (N) and hydrogen (H); both can be hundreds (or even thousands) of part per million (ppm). A rare occurrence, but notable in our case, is boron (B) as an impurity.
A perfect diamond structure.

In black: Carbon atoms. In white: Boron atom. Only one boron atom for every other million carbon atoms is enough to produce a blue color in diamonds. Boron is considered  a (rare) impurity.

If you want to know more about the physics of the cause of color in diamond and in minerals in general, one should refer to this article by Kurt Nassau.

Brown: the most common color for diamonds

Brown diamonds are the most widespread colored diamonds. They were used only a few years ago almost exclusively for industrial purposes, now they are invading the market. Brown diamonds became popular a few years after black diamonds appeared on the market 10 years ago. Brown is still not the most desirable color, as it is not a vibrant color. However, a good marketing strategy made brown diamonds more attractive by labeling them as champagne, cognac or chocolate diamonds, from the lightest to the darkest brown color.
The Wilkinson brooch, composed of champagne, cognac and chocolate diamonds... In other words: brown diamonds! © Smithsonian Institution. Photo: Chip Clark.

Most brown diamonds have a zoning of their color, creating a zebra-like structure, alternating brown and colorless areas (see picture below). When the diamond is properly cut, this zoning is not visible, as it occurs in only one or two directions (in the 111 planes). But when the diamond is purposely cut and polished perpendicular to those features (as below), this so-called "graining" can be revealed. Plastic deformation of the diamond while it was still underneath the Earth surface created these deformation lamellae, in which the brown color developed.

A brown diamond plate, in which the graining is visible. © Gaillou.

The residual stress due to the plastic deformation is easily visible between cross-polarizers, a standard technique used by mineralogists and gemologists.
The same brown diamond plate in between cross polarizers. High birefringence follows the direction of the graining, reflecting the stress remaining in the diamond structure.© Gaillou.

Recent studies revealed that the brown color is due to "holes" (vacancies) inside the diamond structure, with groups of missing carbon atoms (about 60 per group). These groups of vacancies were created during the plastic deformation stage of the diamond. These diamonds can be "cured" by a laboratory treatment called "high pressure, high temperature" (HPHT), removing the brown component, either giving a colorless diamond (for type IIa diamonds), or creating a yellow color (for type Ia diamonds, creating the H3 center). Irradiation followed by annealing treatment would also create a yellow color.
A perfect diamond structure (on the left), and a diamond missing a lot of his carbon atoms in one area, which creates locally a brown color. Extracted from Jones (2009).

Black: the king of pavé settings.

De Grisogono diamond jewelry, set with colorless and black diamonds. © De Grisogono.
De Grisogono panther-shaped platinum bracelet dominated by black diamonds. © De Grisogono.
De Grisogono pearl ring, with black and colorless diamonds. © De Grisogono.

Black diamonds emerged in the gem market at the same time the Swiss jeweler De Grisogono opened his company in 1993. Just like brown diamonds, black diamonds were not used in the gem market before then, but directly sent for industrial use. Now, black diamonds are often use in pavé settings (a lot of small diamonds decorating jewelry pieces), as it contrasts perfectly with colorless diamonds. It is only rarely used as a center gemstone (such as below).

The Spirit of De Grisogono, a 312.24-ct black diamond. © De Grisogono. 

The most amazing thing about black diamonds is that they are never really black! Sometimes, it is the presence of numerous black or grey inclusions (graphite, sulfide, etc.) or fractures that makes them look black. With a microscope, one could see that the diamond is colorless or brown... or any other color, just heavily included, so that the light can not (or hardly) go through the stone. Another reason diamonds can be black is the high saturation of the color, such as, typically, a deep brown, red or green color. In this case, when the diamond is looked with a strong fiber optic source behind, the color can be observed.

Nowadays, some diamonds (mostly brown) may be treated in laboratory to blackened them. It can be done, for example, by strong irradiation of the stone.

White: the essence of opalescence.

Commonly, people refer to colorless diamonds (they don't have any color and they are transparent) as "white diamonds" (which really refers to white opaque or semi-opaque diamonds). They are wrong! Look at the pictures below, and you will most likely see the difference between these diamonds and the diamonds you had in mind.
Two small white (or "opalescent") diamonds.

The whiteness is due to the presence of numerous tiny inclusions, just the right size to diffract light, creating this milky, or "opalescent" appearance. If you put a light behind such a diamond, it will appear milky orange. Also, sometimes, the presence of a strong bluish fluorescence can cause the diamond to look opalescent.
The Royal Butterfly brooch from the National Gem Collection. The wings are made of a pair of yellow (bottom wings) and grey slices of diamonds, and a pair of white and grey slices of diamonds (top wings).© Smithsonian Institution. Photo: Kenneth Larsen.

Yellow: the vibrant Canary color

The Bird on a Rock: The 128.54-ct diamond owned by Tiffany & Co. This extraordinary yellow diamond comes from the Kimberley mine in South Africa, and was purchased rough by Charles Tiffany in 1877. © Tiffany & Co.

Only the saturated (fancy) yellow diamonds are valuable. The origin of the color is well known, and in most cases due to the so-called N3 color center, involving a group of 3 nitrogen atoms surrounding a vacancy (see below).
 In black: carbon atoms; in white: nitrogen atoms. The N3 center is the color center inducing the yellow tint in most yellow diamonds.
The 18.24 ct Shepard diamond, in the National Gem Collection. © Smithsonian Institution. Photo: Chip Clark.
The 253.7ct rough Oppenheimer diamond. © Smithsonian Institution. Photo: Chip Clark.
The Lady Zahira, a 2.26-ct Fancy Vivid Yellow diamond heart is embedded in a rose gold encasing.© Sophia Fiori.

As nitrogen is the most common impurity, it is very easy to grow yellow synthetic diamonds. Actually, it is much more difficult to grow a colorless diamond than a yellow diamond, due to the fact that nitrogen is pretty much everywhere in our atmosphere. Also, natural pale-yellow or brown diamonds may be treated (by irradiation, then annealing) to enhance the yellow color.

Green: naturally irradiated.

The  most famous example of a green diamond is the Dresden Green, in display in the New Green Vault in Dresden, Germany. We visited it in August 2012, and I have been transcended by the diamond's beauty. The green color exhibited by the diamond has a steel tint to it that makes it, at least to me, one of the most special diamonds I have seen so far. The pictures below don't really do it justice... It is a must see!
The Dresden Green, the biggest green diamond to date (40.70ct); it is a type IIa diamond. © Pricescope.
Another picture of the Dresden Green in its setting. © Famous Diamonds.

The green color comes from a color center (named GR1, related to vacancies) that is produced by irradiation of the diamond, when it was still in its host rock deep inside the earth. In most cases, rough green diamonds only have a thin outer crust that is covered with green irradiation spots, while the rest of the diamond is colorless, or another body color. Cutting and faceting such a diamond would remove the superficial green color. The irradiation in this case is due to alpha and beta particles.

To get a homogeneous green color, gamma rays or neutrons are required, which have a much greater penetration depth. These processes can also happen in nature, as the Dresden Green diamond illustrates. Most likely, the diamond has been in contact with a U-rich mineral like uraninite in its early life.

As green diamonds are rare, their price can be pretty impressive... For example, this 2.52 ct Fancy Vivid Green was sold at Sotheby’s auction for $3.08 Million in November 2009.© Sotheby.

Irradiation can also be done in a laboratory... and unfortunately, it is hard, and sometimes impossible, to tell if the irradiation is natural or not.

Blue: the "purest" diamond?

The 31.06-ct Wittelsbach-Graff (left) and the 45.52-ct Hope diamond.© Smithsonian Institution. Photo: Chip Clark.The Hope diamond is on permanent display at the Smithsonian Institution, and the Wittelsbach-Graff is now in a private collection.

We will refer the reader to our blog post published about the Hope diamond to get a comprehensive understanding on blue diamonds.
The origin of the blue color is well-known, and is due to the presence of the chemical element boron (B). The boron present in blue diamonds is in extremely low concentration (1ppm or less). Also, for the diamond to be blue, the concentration of other impurities (such as the common nitrogen atoms) has to be less than the one of boron, making those diamonds virtually the purest diamonds of all! These diamonds are referred as type IIb diamonds.
The Hope diamond as we are used to seeing it, in its Cartier setting. © Smithsonian Institution. Photo: Chip Clark.
The 30.62-ct Blue Heart diamond out of its ring setting. It is on permanent display at the Smithsonian Institution. The Blue Heart has the most gorgeous blue color of all blue diamonds I have seen... Yes, I prefer its color to the Hope, but that's only a personal preference! © Smithsonian Institution. Photo: Chip Clark.
Blue diamonds are extremely rare, especially when they don't have a grey component, and their the color is intense. For example, this 7.03-ct Fancy Vivid Blue diamond was sold for $9.48M at a Sotheby's auction in May 2009. © Sotheby.

Sometimes, a less desirable grey component is seen is present in natural diamonds. The grey color might be due to plastic deformation. This grey color may be removed with a high pressure high temperature treatment (HPHT) in a laboratory.
Also, it is possible to grow synthetic diamonds doped with boron. In this case, such diamonds contain much more boron than the natural ones.
Implantation of boron on natural or synthetic diamonds is also possible, producing a blue and boron-rich "coating" of the diamond. So far, I have only seen a few geeky colleagues, (not to cite any names), to own such diamonds!

Pink: when deformation adds attractiveness and value.

This 5-ct Fancy Vivid Pink diamond was sold for $10.8M at a Christie's auction in December 2009. © Christie's.
The same 5-ct pink diamond put into perspective. © Christie's.

Pink, red and violet diamonds are the most expensive diamonds on the market right now. Their color is related to some plastic deformation that happened in the early history of the diamond, when it was still deep inside the Earth. Diamonds can accommodate plastic deformation by re-arranging their atoms, creating twins (the process is called "mechanical twinning"), as shown in the picture below (Gaillou et al., 2008; Gaillou et al., 2012), and also creating an unexplained color center creating the pink hue. Similarly to brown diamonds, the color is not homogeneous but segregated in lamellae or bands.
The color center inducing the pink color is not perfectly constrained, but recent studies showed that it most likely involves a defect composed of nitrogen atoms associated with vacancies (Gaillou et al., 2008;  Byrne et al., 2012Gaillou et al., 2012).
A slice of a pink diamond from Argyle, Australia. The zebra-like structure, with alternating colorless and pink bands, are easily seen on this sample. © Gaillou.
Another case of pink diamond with discrete pink lamellae in a colorless diamond matrix. © Gaillou.

Each pink band or lamella is made of multiple "mechanical twins", such as this one shown here in this transmission electronic microscope image. The arrows point at the twin plane. Each white spots is one carbon atom. © Gaillou et al., 2008.

The range of pink diamonds: scale to grade Argyle (Australia) pink diamonds.© Argyle Pink Diamonds.

The Lady Leilani, a 1.73 Fancy Vivid Pink heart shape diamond is set in a modern rhythmic white gold tear drop pendant with pave white diamond accents. © Sophia Fiori.
The 5.11-ct Moussaieff Red diamond.© Moussaieff Jewellers.

Among all these pink diamonds, a pure red diamond is the rarest of all. The Moussaieff Red (5.11ct) is graded Fancy Red, and is the biggest red diamond known. The Lady Heart Red is smaller (1.71 ct), but has the best grading possible: Fancy Vivid Red.

For the spectroscopists out there who are wondering what's happening with the violet diamonds... Well, it's just a combination of pink and blue! The transmission in the blue is due to the absence or the weak presence of the N3 center.
UV-Visible spectrum of an Argyle violet diamond. © Van Der Bogert et al., 2009.

Note that it is impossible, so far, to grow synthetic diamonds that replicate the natural pink diamonds (with zebra-like pink zoning, due to deformation). However, it is possible to treat some specific (type Ib) synthetic or natural diamonds to create a color center called "NV center", which produces a pink color. But these diamonds are easily distinguishable from the natural ones.

Orange: the rarest of all?

In one of my reference books on diamonds, "The Nature of Diamonds" (by G. Harlow), the orange color has been described  has "probably the rarest color found in diamond" with a "lists of buyers waiting for stones of this exceptional tint". A pure orange (such as the Fancy Vivid Orange heart shape diamond of the Sophia Fiori collection) does not contain any brown component.

The 5.54-ct Fancy Vivid Orange Pumpkin diamond. © Winston.
The Lady Orquidea, a 2.0-ct Fancy Vivid Orange heart shape diamond is mounted in a heart shaped bezel accented with pave white diamonds.© Sophia Fiori.

I would definitely say that pure orange diamonds are the most mysterious of all diamonds, as they have never really been studied... or at least, if so, the studies have never been reported. Only 2 short publications mentioned a description of the Pumpkin diamond, but no spectroscopy results were provided. In the "Nature of Diamonds", Fritsch indicates that the orange color comes from the presence of a broad band centered at 480 nm (in the blue), but its nature has never been explained.

Grading colored diamonds.

I will refer the reader to this website, which has a great explanation about the terminology. I think that this chart from the same website explain it very well. The most desirable grades are Fancy Intense and Fancy Vivid, the latter being the rarest.
The scale and the 9 different grades for colored diamonds. This chart is applicable to the nominally 27 colors. While "Faint" is the closest to a colorless diamond, Fancy Vivid is the rarest and most desirable grade. © Novel Collection. 


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